As far as I know, SO only offers forum-voted ads for free for Open Source projects (nothing wrong with that, on the contrary, that's great).
But I wonder: would it be interesting to open something like 5-10% of ads to commercial ads ?
Don't get me wrong, I'm not talking of yet another flashy viagra spam advertisement, but there are other non-opensource projects, services or softwares that would be relevant (and I dare even say interesting) to SO users and would be glad to support the site by paying for ads.
You could still require that the advertised service/product be relevant to programmers and has a high enough graphical quality, yet get some extra cash to support the StackExchange network.
What do you think ?