Like most opensource developers I work on my projects because I have an itch to scratch. Apart from the time I put in the only cost to me is the cost of running my website and beers while I type 
Recently there has been a lot noise around Googles Summer of code. Basically Google will provide a $4500 award to each student who successfully completes a project by the end of the northern hemosphere summer. It’s a great idea. It introduces younger developers to the opensource movement and provides a new developer, witch specific goals, to the project team.
Summer of code doesn’t help me directly though. My projects are not up there with those from organisations such as Apache or CodeHaus. On the other hand Google constantly contributes financially to mine and many other projects. The key is adsense
The only real out of pocket expense I have running my projects is hosting. “Hang on” you say. “Can’t you just host your projects at sourceforge or some other community site?” Well yes I could, but I don’t want to. Sourceforge provides a fantastic service and the opensource world has a lot to thank them for but for me they don’t provide the solution I want. For example if I want to be able to run servlets or a subversion server the options just aren’t there for me. So I host on a standard provider (prokmu in my case) and configure things in the way I want them. It’s all about choice
Of course I have to pay for it and that’s where adsense comes in. I (and I’m sure many opensource developers) run adsense ads on my blog and project sites as a means of generating enough revenue to cover my hosting costs. The few clicks I get a day cover my basic hosting which is a nice, all be it indirect, contribution from the good people at Google. So next time your working your way around some developers site or blog and you notice something interesting in an ad, give it a click. You might find something you want and the developer will thank you for the reduced effective hosting costs.