Creating an Open Source Web App that is also a Startup?

I've been kicking around some ideas for a while of some web apps I'd like to build. They mostly relate to social media in such that they would utilize API's of some sites (Twitter, for example) to do data analysis or things along those lines (intending to keep it legit, not "spamware"). Basically, these are things I'd find useful myself to just have to use but that I think could also have potential to be turned into a business at some point. I'm not real gung ho about finding a partner and making a formalized startup, rather I just want to hash out some design docs and start coding, and it'd be nice to work with other developers on this.

Since turning it into some mega-successful startup isn't the goal, I'm fine with the app being open source and then maybe potentially branching it off at some point and adding some new features to try to make money with it, and would be fine with other people doing the same. I'm curious if anyone knows of other examples of this or if there are internet communities more geared towards this. It seems a lot of open source projects, at least the ones I've seen, are more geared at providing frameworks and libraries, not real functional web apps. This is great, of course, as these things are essential, but I sometimes wonder if there aren't other possibilities with open source. Thanks for any info to point me in the right direction.