One of the most unique and fun apps for Android is Remote Droid - the wireless keyboard and mouse for your computer. A small service client must be started on the host computer first - no installation required. It shows the computer's IP address, which then has to be typed into the phone. The connection works through WiFi, so no worries about data charges.
When started, the only thing on the screen are a few colored lines, which represent the trackpad area and the mouse buttons, found on every laptop. Combine this with G1's keyboard and you receive (almost) fully functional input devices. The mouse area is very responsive, supports double click and right click, while the keyboard works fine, too. Some of the many potential uses for this app is to control your home entertainment center, change the song on Winamp or even make a prank to a friend - by making him believe that his computer is possessed.
Worth installing? Even just for its sheer entertainment value, this app is a must for every geek. A candidate for the "Best Android App" award.
What do I want to see in future versions? Multi touch gestures - whenever Android supports it.