Some are outright villains, using their unique skillset to corrupt good men, start wars and frame others for their deeds. The other kind is a little more ambiguous. They may appear to be evil, or at the very least self-serving, but at the core of it all, they're actually on the side of good, even if their methods are questionable.
This list includes examples of both types, with points being awarded for ambition, deviousness, sexual potency and overall style.
5. Clytemnestra

The Trojan War lasted ten years and then the journey back to Greece took some time as well. During this time Clytemnestra conducted an affair with Agamemnon's cousin, Aegisthus, while she plotted her husband's demise. Apparently she hated him even before the sacrifice of Iphegenia as he had murdered her first husband and forced her into marrying him. So Agamemnon finally comes back from Troy, with hot little Cassandra in tow as his concubine and Clytemnestra makes her move and stabs him to death in the bath.
This left Aegisthus as the new king with Clyemnesta as queen. They ruled happily for awhile but eventually she's murdered by one of her own sons in a power move. Like mother, like son.
4. Phyllis Dietrichson

The introduction of Phyllis Dietrichson in the 1944 classic Double Idemnity remains an extremely memorable scene in American cinema. It has most of the hallmarks of a film noir scene - the shadows of drawn shades are visible in the dim lighting, and a seemingly innocent conversation plants the seeds for not only an intense attraction but also a diabolical plot. She appears calm and cool and doesn't do or say anything overtly flirtacious and yet Neff is certainly left with a firm impression.
Phyllis's motives soon become clear and her seduction, both through the lure of money and her own sexuality, escalates. At first Neff wants no part in a murder plot but very quickly he succumbs. Shortly thereafter a formerly good man has become a murderer. Once the victim's daughter becomes a potential problem, Phyllis displays no qualms about knocking her off as well. She's proven herself to be quite the ruthless character. In fact, it's strongly implied that she had murdered her husband's first wife - and the girl's (Lola) mother - to get to where she is. But wait! There's more!
It turns out Phyllis has also been using her feminine wiles on the girl's own boyfriend and is still involved with him at the time of her partnership with Neff. It's only once he finds out this detail that Neff finally realizes just what a monster the woman he's become involved with is. Once confronted, she promptly lies (presumably) and when this doesn't work, shoots Neff. It's not a fatal wound and Neff comes on and manages to get the gun away from her. She insists her love for him is genuine, citing her inability to fire again as proof. But at this point Neff isn't buying it and he shoots her twice, killing her. But the damage is done. Neff will soon bleed out, an innocent man is dead and a girl has now lost both her parents. All because of one lousy dame's greed.
3. Morgan le Fay

In most versions of her story, le Fay is said to be the half-sister of Arthur as they share the same mother. But as to her personality and motives, that seems to vary somewhat depending on what you read. Some of the earliest stuff portrays her as an ally to Arthur, who uses her powers to be a great healer. It's in the French Lancelot-Grail that her character becomes much more complex and interesting. Here she's an antagonist of Arthur and his Knights of the Round Table and uses her magic and powers of manipulation to defeat them. In Thomas Malory's famed le Morte d'Arthur it's actually she who throws Excalibur's special scabbard into the lake after a failed attempt to steal the legendary blade.
She also figures prominently in tales of Charlemagne as an enchantress and seductress, sometimes aiding the heroes by testing them and others opposing them more directly.
Her name and many of her character traits have endured over the centuries and she's been used again and again in various works of fiction including the Marvel and DC universes, many movies based on the Arthur Legend and even the anime Ah! My Goddess. Throughout it all she's remained a femme fatale of the highest order.
2. Ada Wong

She first appears in Resident Evil 2 in Leon's scenario and it quickly becomes apparent that there's more to her than she's letting on. But if you're a gamer who pays attention you'll know that she is actually mentioned in the first Resident Evil (set in 1998), in a letter written by a dying Umbrella researcher named John. She'd used her considerable feminine charms to convince John to try to steal secrets from his employers. Ada requires information to pass on to her own employers - referred to only as "The Organization". He is among the infected during the first T-Virus outbreak in the Spencer Mansion in the Arclay Mountains outside Raccoon City and in his letter, pleads Ada to destroy the mansion and expose Umbrella's actions to the public. Unfortunately, the letter never reaches her.
Months later, during the outbreak in Raccoon City, Ada is sent by the same unnamed organization to try to steal a sample of the new G Virus from the secret Umbrella lab beneath the city amongst the chaos. It is here that we first see her, confronting her as Leon. Her story is that she's looking for her boyfriend, "John", whom she claims is a journalist. Her agenda is eventually exposed but she still displays heroism by saving Leon, which in turn, helps him rescue Claire Redfield and Sherry Birkin.
Spinoff titles give us a little more info on what she was doing during RE2 and she next plays a major role in Resident Evil 4 (taking place in 2004), once again working against the game's main antagonists (this time Saddler and the Los Illuminados cult) but with goals separate from, and sometimes counter to, Leon's. Once again she proves helpful but still self-serving. Her flirting with Leon is really amped up this time around and her skills as an operative are on full display, particularly when you play her scenario. Despite working against Wesker (her new employer) by helping Leon kill fellow operative Krauser, she still delivers the goods in the end - the Las Plagas sample. HOWEVER, if you check out "Ada's Report" you discover she was playing Wesker as well, giving him the lesser sample, and keeping the dominant one for herself.
In Resident Evil 6, Ada is still sexy and dangerous in 2013 and I think her actions there cement her status as a crazy awesome femme fatale. You never really know what side she's on besides her own but she's still benevolent enough to risk her neck to help others if the situation calls for it. Antiheroine she may be, but if you really don't think she's also a femme fatale, maybe you should give that sultry voice another listen.
1. Catwoman

As I said in the intro, femme fatales have been around about as long as human beings have been on this planet but it was Selina Kyle who finally perfected the art. First appearing way back in Batman #1 (note: this is not Batman and Robin's first appearance, as comic book sages like myself know - that would be Detective Comics #27) in the spring of 1940, she's been a major part of the Dark Knight's mythos ever since although she actually disappeared from all Bat books between 1954 and 1966 - most likely because her moral ambiguity paired with her overt sexuality would have made her a prime target for the Comics Code introduced in the mid fifties. This was easily a low point in Batman's publishing history, as well as many other comic characters, both major and minor. Most stories were lighthearted and goofy fare, moving away from Batman's dark roots and it's not at all surprising in retrospect to note that Catwoman wasn't present for any of it.
Anyway, once Batman returned to his roots and the comics started getting good again, everyone's favourite cat burglar-turned-sometimes-crimefighter really began to shine. She's become such an enduring symbol of a femme fatale that most people who haven't even read any comics know that Catwoman is a bad, bad girl. But still good.
Like Ada, she makes for a pretty good antiheroine, always playing it cool and letting others know that she plays by her own rules and will only risk her neck if she sees some profit in it - although at times we've seen she can be heroic just for the sake of doing the right thing (but she'll never admit to it).
From the sexy costume to her endless flirtation with Gotham's staunch defender (she's about the only character who can really throw him off his game - quite a feat when you consider the types of characters he often deals with) to her overall badassness, Catwoman is the femme fatale that all others should look up to.