AirPort: Apple Computer's 802.11b (currently - could change in future?) that is standard 1-2-5.5-11 Megabit transmission over the 2.4Ghz frequency spectrum. Depending on which country you live in, it can use up to 13 (11 in the US) channels - akin to CD radio channels, though none are reserved for the police :-).
The sell a BaseStation for about $300 - which includes a 56k modem, a KarlNet based 486 PC with 4 MB memory (flashable), and a Lucent/Orinocco/whatever Silver (40/64-bit WEP encryption). The BaseStation is an AccessPoint - and a hackable one at that!
You can add a different 802.11b (other PC-Card? g? a?) card for more encryption ("Gold" -> 128-bit WEP)
- You can add a Lucent external antenna to increase the range, or any other 2.4 GHz antenna that has a Lucent plug
The AirPort client cards sell for about $100 from Apple, and fit in any modern Mac CPU - G4 Towers, iMacDV's (400 MHz or newer), any iBook, newer G3 PowerBook and any G4 PowerBook.
They've got a pretty detailed consumer-level description at: http://www.apple.com/airport
Modifications for the Base Station can be found at: http://www.vonwentzel.net/ABS/
N.B. The above refers to Apple's AirPort base station v1, aka "Graphite" (they're gray). Apple released a new version of the AirPort, v2, commonly referred to as the "Snow" base station because of its white case. Snow base stations are completely different from the original graphites -- instead of 486s running KarlNet, they are reputed to have PPC innards. Little is known about their firmware, and they are not (yet) known to be hackable. It is easier to add an antenna to a Snow (less case mangling required).