How to build a simple LED transmitter and receiver
There are several optical free space communication designs as part of an open source project called Ronja (Reasonable Optical Near Joint Access). They have plans, schematics and software published under the GNU Free Documentation License. They claim a 1.4km range with a stable 10Mbps full duplex data rate, the system uses either IR or visible light LEDs with focusing optics, and interestingly does not seem to use any spectral filters.