I have a reasonably successful open-source project ( www.mediabrowser.tv ). The website is seeing over 50K unique visitors a month.
I would like to monetize some of this traffic by offering a "sponsors" section on the site and perhaps some sponsor logos in the footer.
What kind of a monthly fee can I realistically charge for this given I am getting 50k unique visitors a month? Naturally sponsors will be in the same "space" so by having a spot on our site they will be able to reach a fair amount of new business.
Why don't you start with something like Google AdSense and measure the results?
Why don't you put a placeholder that says "Sponsor us" and let potential companies make an offer.
You have a good number of uniques - what about returning visitors?
I think in this case, you have a product that end users want to download and can use without the need of returning to the site. This makes it difficult to do any sort of banner ad campaigns via a network.
If a significant number of your traffic is found within the forum, I would consider getting a phpbb mod to support ads and put some house ads in there.