I've known a few over the years, they aren't as rare as we might think. The ones I met since a couple years are playing MMORPGs (coincidental with the amount of time I've been playing MMORPGs myself), often on laptops for some reasons (not sure why but "PC gaming" for many of them seems to be synonym with mobility and low performance), or if not they are console gamers (whom happen to play on-line anyhow via XBOX Live or PSN when they do play on-line). I've started playing DC Universe Online recently, and on the Leagues recruitment forums there's a thread for an all-women League ("League" in DCUO being a guild), although I don't know if all of their members are indeed girls in real life. As I said most girls I know of whom play games just happen to be playing the same MMORPGs I'm playing myself (mostly F2P ones). I do know when the player is indeed a girl only when I get to hear them talk over their microphone during any given game event (in-game or on ventrilo for example).
What I wonder myself though is how many are off-line gamers, and how many are on-line... and why not how many are doing both. I remember discussing that (video gaming and girls, in general) with a friend. She, a gamer herself back then, said that us "boys" started realizing [more] that girls do play and like games simply due to the advent of on-line gaming over the years, simply because many gaming girls went with the trend and moved from off-line gaming to on-line. It's true that a few years back (or longer perhaps, a decade at least) during the years of 8-Bit, 16-Bit and 32-Bit gaming that we wouldn't get to know them as much as we can today, since back then gaming was done off-line and at home, for the most part. Nowadays if you play exclusively off-line or if you're content with playing games on your older consoles only occasionally then of course no one will know, but then again the girls who belong to the "off-line" category don't actually care either.
As a side note, I've met one girl whom happened to be playing WoW very often back then (that was... around four years ago) and yes she was extremely charming and svelte (we stayed friends... dammit). She often spoke of her latest raids and how fun (or not) it was, etc. And of course I tried to stay interested and smiled as she explained how it went, but she had no idea how I couldn't care one bit about it [that I never played WoW back then nor that I ever cared about it]. I never told her, being the gentleman I am, c'mon guys! But anyway, as of now the girls I know of are playing MMORPGs and we (me and they) just happen to be in the same guilds, sometimes I get to hear them talk as we play around. To be honest though they are far more numerous (at least in those games) than I originally thought possible, I'm kinda shocked really.
There's still this stereotyped years-long "nerdy-manized" (my cousin came up with that one) perception in my mind that gaming in general is indeed still mostly experienced by "guys" (I.E men, or if you prefer pubescent and per-pubescent perverts alike), and that girls (I.E REAL girls, not a "Guy. In. Real. Life") are but a mere product of our imagination (or desires). Please, girls, don't blame me, I've grown from those years of video gaming during which it was common belief that if you ever dared touching a console's controller that you'd die a virgin, and that revealing your gaming habits to any girls would be a guaranteed failure for a potential relationship (total turn-off, "never tell a girl you're a gamer!").
Ah, yes... the good ol' days.