[petercooper]

You could get creative, but it'd definitely involve spending some time.

I'd use the space myself for building an e-mail list, Twitter following, Facebook following, or similar which could then be used to make money further down the line. I don't know much about your game mod/project but if everyone going through that page is an avid Steam user and gamer, if you could get 100k people following on Twitter, on an e-mail list, on Facebook, etc, that's a very valuable audience for many people.

TL;dr - Don't let third parties advertise on the page, use the space to build your own audience, then sell advertising/make money on the back end.