I've started a few open-source projects on Sourceforge. The only one that got other people to contribute was one that (a) had functionality valuable to commercial developers and (b) there was no commercial product available that had the functionality. Basically, it was a niche that had not been commercialized. The people who found it did so by Googling for certain terms.
So, for me, getting people to contribute was a matter of doing something "unique."
BTW, once a commercial product became available that did what my project did, the contributions stopped. I guess it was easier to buy the thing than to help build it.
