2.4GHz parabolic mesh dishes from Chinese cookware scoops & a USB WiFi adaptor! Called "WIFRY","WOKFI" or "WOKTENNA" (12"= 300mm diam) shows 12-15dB gain (enough for a LOS range extension to 3-5km)
Weekend Project Podcast: Make simple radios to listen too. These were made with materials that they could get their hands on and were small enough to carry around in a big pocket.
Nice little guide to building an IR transceiver - uses the USB port for stable 5V, but includes a link in the comments to a decent circuit diagram that uses a voltage regulator and therefore doesn't need the USB port for power.
A remote control car that can be driven over the internet or with a laptop wirelessly from up to 500m away. It has a live-feed network camera so that it can be driven without line of sight and a horn so that you can honk at people.
"If you have ever watched a TV quiz show you have probably seen contestants trying to press a button in order to win a chance to answer a question. The contestant’s quick reaction time results in some kind of light and/or sound indicating victory. This is a practical way to choose the next focus of the game’s activity and it adds a bit of excitement to the process. So when the holiday party planning committee decided to have a trivia contest I decided to build a quiz contestant lockout system to add an extra dimension of fun to the festivities. This would help the planning committee’s mission of creating some entertaining activities for the event."