All American Singles Internet Dating
AmericanSingles.com membership: this site description is self-evident. It's free & geared for the generic person.  The site is nice in that it loads people's pictures quickly and you can control for location. Unfortunately, to know anything beyond a person's age & looks you must read each person's profile. The good news is that there are more than 3 million members!
DatingClub.com A solid basic dating service. It even allows you to register in different languages!ePersonals.com Turn your love around. it's a new dating site that works.
Iwantu.com: is a good dating site, but a little hard to use. geared for all types of people
Howtohavegoodsex: Over very own brand new dating site that is free to send and receives messages. Definitely worth checking out. 
LoveCity.com: Your basic generic site. It's worth looking at, but not one of the best. 
Match.com. Try it free for 3 days!: This is my favorite site! I can't say enough about it. You can control for everything, age, religion, education, location and people write useful essays that actually describe themselves.
Mingles.com: the relationship network.
PlatinumAlternative.com a basic site.
Seeking you: is one of the best sites. while there are not too many people signed up yet, it has search functions which allow you to choose the telephone code area, religion, education and much much more. this is definitely one of the best sites out there for mainstream American's. 
Ucandate.com: this is a basic Internet dating site.

Christian Sites
Bigchurch.com: A huge Christian/Church community and dating service - thousands of active members looking for friends, marriage, even bible-study partners. The site has a searchable bible, bible emails, chatrooms, and more.

Swingers
Adultfriendfinders.com has the largest swingers directory that I have found.

S&M
Alt.com: is a site for people interested in finding a partner who also enjoys the S&M world.  

Seniors
Seniorfriendfinder: this site is geared for older people. It's an excellent way for older people to get to know one another!

Categorized by Sports 
Sportsfriendfinder: this site is dedicated to people who are active in sports.  Get ready to play!

Sites Dedicated by States
Dateable.com: is unique in that it is really three different dating services.  The first service is only for people living in the NY tri-state area.  The second service is for people interested in dating someone of the same sex. The third service is geared for people who are interested in an 'alternative' sex-life.   

Asian men & women
Asiafriendfinder: is a site geared for Asian men & women.
Cafefusion: another site dedicated to meeting Asian men and women. .

Internet Dating sites that allow you to register in different languages
DatingClub.com A solid basic dating service. It even allows you to register in different languages!

Singles Tours
A Pretty Woman

Brides to order
Brides to order