I'll add to this list from time to time, but for now, some good places on the web to start looking are:

The National Organization for Women

Though not the only organization dedicated to the rights of women, NOW is one of the most easily recognizable organizations devoted to women's rights. Good general overall information, with a focus on activism.

The Feminist Majority

This website has both an easy to remember URL and one of the best collections of links to other good sites on the web. Their links are organized around themes. If you're interested in clarifying the definitions of Third Wave Feminism, this page has some good links to self-identified Third Wave sites.

Ms. Magazine

A feminist magazine. Our library has hard copies as well.

Bitch Magazine

A slightly more third wave leaning magazine with a pretty irreverant attitude, but which has some excellent articles as well.