TrendNet TEW 424UB Wireless USB adapter.
I've had mine for a while now, a bargin at $20 a while ago, seems to be found for between $15-25 these days.
It has a few flaws that people complain about, mostly relating to it over heating which causes it to do a memory dump and blue screen your computer, especially noticeable with Windows Vista for some reason, even using the latest Trendnet drivers, or using the Realtek generic drivers, no difference.
I had this problem happen to me a fair bit, but I discovered it is easily fixed. It comes with what appears to be a useless extension that the device plugs into and which then goes into a usb port, this puts the device about 5 inches further out. Bending this flexible extension into a "L" shape allows the device to free stand in the air, away from any surface. This has kept it cool, and I've not had a crash with it since doing this.
It doesn't like being plugged into the back of the desktop if there isn't any air flow, nor does it like resting against the desk if plugged into a USB hub, which was how I had it set up, but now free standing in the air, it's golden.
The speed of the device is consistently good. The software included with it is basic, but it works, and you really don't need it with Windows anyway. Certainly very easy to connect and get working.
I'd recommend it as certainly good value for money if you need a simple wireless connection for your desktop/laptop.