Google docs?

Blog?

Starting a thread here?

Asking for playtesters and sending a document to people who request it?

Starting a thread here, asking for playtesters, and generally asking pointed questions in several places. (Other forums, of your playtesters or asking other  designers..)

Show stuff around on the internet. You can get all kinds of useful advice.

Pick up multiple playtest groups. Ask one or more of them to play on AIM or something like that, so you can read the chat logs and see what a session looks like in detail.

Ask specific questions.

If you're looking for playtesters, my experience is that outside of people you meet face-to-face (I've done some playtesting at cons, for example, where I can interview afterwards) you generally won't get much information back. All too many people just want to collect your game on their hard drive.

If you have a budget for it, pay for commentary. Even if you just promise a comp copy or an interior credit, offering something for commentary is more likely to get you something useful. If you have a form to fill out, that sometimes helps.

Forums like this are very hit and miss -- topics are all too easily derailed into rants about "what I like" or worse, "what is a role-playing-game" -- so I wouldn't bother except in places where you already know you have a half-dozen reliable and thoughtful commenters. I won't aim you at specific forums because you'll have different results than I will.

Thanks everyone.

I'm asking because I'm getting close to finishing my online game, and my next project is a worldbook set in the same world.

I have found the best feedback is paying attention to the players during the playtest game.   This is easiest if you are not the GM, but usually you will be.  

I am always interested in noting what mechanics trip up players, what parts of the setting they discuss during play after my initial setup discussion and what bits and pieces get the most attention during play.  

Afterwards, I do Q&A session where I ask for thoughts about specific bits, but not the "do you like it" questions, but "what did / did not work" and why.

I like to discuss "character concept vs. mechanics" because I am focussed on the idea that Character XYZ should be able to do his XYZ stuff in the game from 1st level (or whatever equivalent) onward.  

My second best feedback was by submitting my RPG to the PolyCon Game Contest which was great fun - especially because I won.  But more importantly, I got 8 players and 2 GMs who I did not know to judge the merits and flaws of the game using the same criteria they used across the board for other submissions.