http://www.adventuregamestudio.co.uk/
On this site are a huge bunch of games that have been made mostly by amateurs just for the love of it, but some are very well done and great fun to play. They're free to download, so I hope they fit your criteria.
Edit:
These aren't all adventure games. They have a section for non-adventures, and I noticed some sport titles there, such as FIFA International Football 2004.