I'm willing to be a beta reader, and I will be looking for beta's as well in the near future. I read fantasy, SF, horror, mystery, supernatural, and YA if it falls into one of these genres. I'm not particularly prudish about sex, but I doubt that I would make a very good reader for Erotica (no offense intended).
UPDATE (9-1-07):
1. I don't do line-editing. Grammar, spelling, and punctuation
is are no, is ...
are? your job should be checked by the author. I've got enough of my own to do.
2. Don't send me your manuscript until you are certain that it is written to the best of your abilities, and ready to be presented to an agent or publisher. I don't want to help you write it, I just want to help you make it better.
3. I prefer to read the entire manuscript, rather than batches of chapters or scenes.
4. I may respond via a steady flow of email messages as I read each chapter or section of your story, or, I may read the entire thing and send one big email at the end. The latter usually occurs with manuscripts that are very highly polished, and about which I have very little useful advice to offer.
5. When I read, I am primarily concerned with all the major story elements: plot consistency and plausibility, pace and rising tension, POV shifts, character development, etc. blahblahblah, and so on. If you have some specific issues that you want me to pay particular attention to (head-hopping, for example) be sure and let me know.
6. If something about your story really impresses me, I'll mention it. Otherwise, I'm not one to hand out a lot of compliments and praise. I'm more of a "just the facts, ma'am" sort of beta reader.
7. I cannot, at this point, legitimately call myself a professional writer. I have been writing, and studying the craft of writing, for nearly twenty years, and of course, I am an avid reader. I believe that I am qualified to help fellow writers improve their stories--else I wouldn't offer to do it--but of course, all I really have to offer are somewhat biased, largely subjective opinions. If something that I point out or suggest clicks with you, well, alright then. On the other hand, if it sounds like I am talking out of my ass, I may well be. No harm done. Light a match, and let's move on.