Recently there have been additional opportunities for Web Developers in earning advertising revenue through Downloads and Link Shortening.
I have found most to be quite intrusive to the user experience, but I have been looking around and the following two seem a good balance, of pay out and good user experience.
Adf.ly - If you need to shorten your URLs, especially useful for Twitter but you can use it for any kind of link which you wish to Monetize. I have seen it used for downloads also. They will pay you for each visitor which you direct.
Link:
Adf.ly with referral or http://adf.ly/
Hotfile - Upload your files to hotfile and be paid for each download. Advertising is shown, no annoying surveys which are usually difficult to complete. However only paid on files larger than 5 MB, but better payout for larger files.
Link:
hotfile.com with referral or http://hotfile.com/