On the heels of recent 802.11g and 802.11n add-ons from QuickerTek (see "," 2007-08-30), Other World Computing has  that bring 802.11n to Macs that lack built-in 802.11n chipsets. The adapters include a USB dongle, a PCI/PCI-X card, and a CardBus card. All three require Mac OS X 10.3 or later or Windows XP or 2000 and later, including Vista.
The USB dongle will work with any PowerPC G3/G4/G5- or Intel-based Mac that can run Mac OS X 10.3 or later. The PCI/PCI-X card will work only with appropriately equipped Power Mac models, and the CardBus card only with PowerBooks.
The 802.11n standard provides a much greater coverage area and range than its predecessor, 802.11g. Apple added 802.11n into most of its products in late 2006, and provided the enabling software along with a new base station in February 2007. But some current and all older Macs have only built-in 802.11b or 802.11g adapters.
The Other World Computing and QuickerTek 802.11n adapters work only in the 2.4 GHz range, a relatively crowded slice of spectrum that is full of existing Wi-Fi networks, in which Bluetooth hops around, and in which other uses abound. Apple's own adapters work in the 5 GHz range as well, which has fewer current users and usages, and nearly eight times as much available spectrum.
Other World's adapters differ from gear from both Apple and QuickerTek in that they allow the use of so-called wide channels in 2.4 GHz, which is a bit controversial. A regular Wi-Fi channel uses about 20 MHz of spectrum. With 802.11g, it can reach a raw data rate of 54 Mbps, and with 802.11n, about 150 Mbps; that translates to about 25 Mbps and 70 Mbps of real throughput in ideal cases.
Wide channels use 40 MHz, the equivalent of two channels, and double the raw rate to 300 Mbps, achieving rates in my testing of up to 140 Mbps in the 5 GHz band. Apple allows wide channels only in 5 GHz, where it supports 8 of 23 possible channel choices (with more to come, I believe). With only 3 non-overlapping channels in 2.4 GHz, due to the way in which channels are assigned, a wide channel has the potential to step on more networks that might be operating in the same space.
Part of the delay in 802.11n's finalization continues to be defining the rules that keep 802.11n from being a bad neighbor. When 40 MHz channels are in use, 802.11n is supposed to back down to 20 MHz whenever it senses any network activity in the wider range. In practice, that's still being sorted out.
Other World's gear should work just fine with Apple's AirPort Extreme Base Station with 802.11n (the latest version released just last month with gigabit Ethernet; see "," 2007-08-13), but it won't achieve its highest possible speeds unless used with another base station that offers wide channels in 2.4 GHz.