It's funny, but I created the first alt just for testing, then the avatar developed a life of her own! I'd let friends know the alt's name in case I was online as her and they needed to reach me. One friend tp'd me when I was "her" to a neat place he'd found that was running a kind of all-inclusive fantasy RP. Before I knew it, my alt had morphed from a Neko (I was having fun with that already) to a Drow (dark elf). That alt studies archery and magic inworld. Quite trippy.
My other alt was inspired by a friend who was enjoying the sociology of gender role play. He is an actual male (I know him in RL) but uses a female avatar exclusively. Not for any cybering, just for exploring that side of himself. So I've got a male alt now. It has been a really interesting experience -- when I'm "him" my language changes. And playing with his appearance was really a challenge. I wouldn't say he's even at the look I'd like yet. I have to fight the urge to make him look feminine!
I've also used my alts to create a land-owning group. I've saved a few Lindens (the local currency) that way!
Mostly though I log in as my main, who is usually dressed as a fantasy version of the real me: thin again, ginger-haired, hazel-eyed Liz. With whatever neat clothes I've recently found.
