There are many types of alternative lifestyles or niche lifestyles. Finding people who share your values and interests may be easier if you choose a dating site based ong your age, religion, orientation or sexual interests, whether these categories feel alternative to you or not.
The Ashley Madison Agency
The Ashley Madison Agency caters to married people seeking affairs outside of marriages. Most of the people on the site are married, but some will be single and looking for an affair with a married person.
Poly Matchmaker is an online dating site for people in polyamorous relationships. The clients on this site will identify as a range of sexualities but all be interested in non-monogamy. http://www.polymatchmaker.com/
Christian singles is a site designed exclusively for Christian adults looking for relationships with people that share their religious beliefs. The people on this site are supposed to be "marriage minded" and "have accepted Jesus Christ as their Lord and Savior" according to the website. http://www.christiansingles.com/
Curve Magazine is a magazine geared toward lesbians. It offers an online dating site for women interested in other women. As with other sites, you will create a profile and be able to search for and meet other lesbians. http://curvepersonals.com/
Alt.com is a dating site created exclusives for the bondage and discipline/sadism and masochism community. It is safe to assume that the people you meet on this site will be into some aspects of BDSM. http://www.alt.com/
Senior Friend Finder
Senior Friend Finder makes online dating easier for an older crowd. The personals on this site will all feature men and women of more mature ages. http://seniorfriendfinder.com