Coalition for Positive Sexuality
Excellent information about sex, including sex as a pleasurable activity, not just STDs and pregnancy prevention. Also en español.

teenwire
Lots of good information about teen sexuality and how to build healthy relationships. Games, quizzes, stories and resources. Some portions en español. Sponsored by Planned Parenthood Federation of America.

Advocates for Youth
Great information about activism for youth sex education. Also has great resources. Also en español.

Go Ask Alice
"Alice" answers lots of questions about almost any health- or relationship-related question. Columbia University Health Education Program.

TeenGrowth
General teen health site with medical advisory board. Lots of information.

San Francisco Sex Information
Free and anonymous way to have sex questions answered.

Sex, Etc.
Created by teens for teens. Good info about pregnancy and infection prevention.

Scarleteen
For teen girls from the creators of Scarlet Letters for Women. Excellent sex-positive site. Has realistic information about pleasure and sexuality. Opportunities to share stories with others. Made for all genders but particularly girl-friendly.

My Sistahs
For girls of color. Deals with all sorts of health and body issues, including sexuality and relationships.

Not-2-Late.com
Reproductive Health Technologies Project gives information about the emergency contraceptive and where/how to get it. Also en español and en français and Arabic.

National Youth Advocacy Coalition
Promotes advocacy and activism -- geared toward all, not just youth.

OutProud
National Coalition for Gay, Lesbian, Bisexual and Transgender Youth offers information and resources.