I enjoyed Trillby: The Art of Theft, and while not really freeware, "World of Goo" is about 20 bucks for a download, I haven't regretted it.
I downloaded N, and that held my attention for a little while, if for no other reason than it's so mind-bendingly difficult.
I downloaded N, and that held my attention for a little while, if for no other reason than it's so mind-bendingly difficult.