Hans Summers has an amazing collection of projects. His most popular project is the binary clock. It runs off of mains, uses a bunch of TTL logic chips and a binary counter. He has posted links to the many projects that have been derived from his original post. Warning: project uses LEDs. If the binary clock isn’t your thing he has lots of other clocks, radio, frequency counters, computer and other projects.
[thanks Alan Parekh]