I launched RubyFlow - a public link blog for Ruby stuff - on April 9, 2008. On the same day, 3 other people posted items. The next day, about 10 others. This kept up for a few weeks but it died down a lot so I picked up the slack, did more promo, and since late 2008 it's basically run itself.

The trick? Have a popular site in the niche you can use to promote the heck out of the new site ;-) Stack Overflow is a great example of this technique working on a bigger scale.