Having a projector of your own that works well with your laptop can save time and hassle when you want to set up a presentation at a remote location. Small projectors can be fun for watching movies back at the hotel, too.
If you value portability most, consider the Optima PK 102 ($250, at lower left), which is about as big as a deck of cards. With 4GB of storage and built-in presentation software, it can function as a stand-alone projector for even fairly large presentations. It's bright enough for dim rooms and medium distances; but when we used it to project at distances of more than 6 feet from the screen or wall, it produced weak images.
For a brighter presentation with clear and readable text, the Aaxa P2 ($350, at upper left) is a good choice. You can expand its 1GB of built-in memory via MicroSD, and built-in speakers allow this unit to function as a stand-alone presenter. But the increased brightness comes at the cost of battery life; you'll want to keep your presentation to under 50 minutes.