Sikh American Heritage Organization - promotes and fosters fellowship with the American mainstream and minority communities while maintaining Sikh values, heritage, and identity

Essence of Sikhism - claim to be the largest Sikhism resource on the 'net.

Intro to Sikhism - a good starting point.  Introduces you to the religion, information on Gurudwara's, information on different Sahib's, etc. 

Maboli Systems - original Sikh content on this website.

Sikh Books - a bookstore offering books related to Sikhism

Sikhs.org - The Sikhism home page.  A portal on Sikhism.

Sikhmatrimonials - you guessed it...get a spouse!

Sikhnet - another Sikh portal.  Very well laid out.

Sikhseek - "the Sikhism web guide for finding anything Sikh"

Sikh Names - a site that has Sikh names and meanings

Sikh Study Circle - a site dedicated to educating Sikhs.

Sikh Women - an excellent all in one site

Sikhism Links - a page dedicated to all kinds of Sikhism links, including Bhangra ;-)  Also has links to other religions.