I should note that I am a strong fan of Albel and Nel pairing (from SO3). An Alnel story would have to be really bad for me to turn it down. ^_^
I tend to work within the video game area. RPGs that I've played (preferably more than once) are my forte. Right now, that's mostly Star Ocean 3 ('Til the End of Time), but I'm willing and able to beta for other video games such as just about any Final Fantasy game, Rogue Galaxy, Radiata Stories, Xenosaga, Prince of Persia Sands of Time, and so on. When in doubt, ask! Even for games I am not familiar with, I can still do spelling and grammar checking at the very least, even if I might not have the knowledge to keep the characters in character. You can always have multiple betas for that.
For other genres - my next most 'doable' category would be books. I read fantasy/sci-fi, and I'm familiar with many of the popular 'young adult' authors like Sherwood Smith, Robin McKinley, Mercedes Lackey, Diane Duane, Tamora Pierce, Meredith Ann Pierce (unicorn books only), and Patricia C. Wrede (Enchanted Forest, Sorcery and Cecilia, and Mairelon the Magician, and any Lyra books I can get my hands on). I also read more adult fiction like Anne Bishop's Black Jewels books, Elizabeth Moon's Paksenarrion trilogy, Jim Butcher's Dresden Files, and Elizabeth Haydon's Symphony of Ages. Again, it does no harm to ask! I'm always reading new stuff.
Other things I'm familiar with...well, I've read a few manga - FullMetal Alchemist, Tail of the Moon, Vampire Knight, Crimson Hero, Ouran High School Host Club, and MeruPuri. Oh, and don't forget, my first and favorite one ever (thanks Blue Persuasion) From Far Away!
For TV...well I don't watch much. But I did see a couple cartoons I'd be willing to beta for - Gargoyles and the old Dungeons and Dragons cartoon. For non-cartoons...I don't know. Maybe CSI (the original) or NCIS.