A black rectangular prisim with the proportions: 1:4:9. They are supposedly created as tools by a race of super-evolved beings that exist as pure energy (as explained in 3001). Sort of like a cosmic Swiss Army Knife. I wish I had one, but the user interface doesn't look to friendly.
The monoliths are featured mainly in Arthur C. Clarke's Space Odyssey series (2001: A Space Odyssey, 2010: Odyssey Two, 2061: Odyssey Three, and 3001: The Final Odyssey).