The airport card in my laptop died and I couldn't get it fixed before going away for a week. So I needed a stop-gap solution. There are many USB wifi adaptors on eBay that claim Mac compatibility. Some are super-cute little things that only just stick out of the USB port (how they have enough antenna showing to get a signal is anyone's guess).
They almost all seem to use a Realtek chipset. (You can verify this by going to System Information on your Mac, opening the Hardware>USB page, and selecting the device. The bottom pane shows the device info, included the manufacturer name (look at the Vendor ID line).
If you're lucky, you'll get a driver CD bundled. If you're very lucky it's even got some Mac drivers on it.
Don't even try to use them! They'll be hideously out of date.
And don't go to the Realtek website and download the driver that matches your chip number. no doubt that too will be hideously out of date.
I wasted hours doing that; it reminded me of why the Mac is (usually) a so much better computing environment that Windows and Linux. I wasted hours fiddling with different driver versions, 32-bit and 64-bit kernels, cleaning out old driver installations, Googling forum postings, trying other driver versions, and all sorts of tedious tomfoolery.
The simple solution I found, which works on Lion, with 64 bit kernels and 32 bit kernels, is to install the following Realtek driver:
Select the RTL8188CUS>Others>Mac OSX 10.7 Install Package (UI ver 1.9.7) version.
Even though the chip number doesn't match my USB key, the software is perfectly compatible with older devices. And it has the added bonus of working.
Remember: the driver doesn't integrate into Apple's airport utility. So you'll have to run a separate client to set up the wireless network. It isn't pretty. But it works.