I have an 802.11n router in my house, but not all my PCs and laptops have 802.11n adapters. Most of them run 802.11g, which drags down the overall performance of the network (it's a lowest-common-denominator kind of thing).
Cheap fix: Newegg has the Netgear RangeMax Wireless-N USB 2.0 Adapter WN111 for $13.99 shipped. It's a refurb, but, hey, something like this either works or it doesn't.
To get that discounted price (it's normally $24.99--and new units list for $70!), you need to apply coupon code EMCLSNW25 at checkout. You also need to be a subscriber to Newegg's newsletter.
As you may know, 802.11n Wi-Fi delivers dramatically faster performance and greater range than 802.11g. That's critical if you want to stream music and video (particularly HD video) around the house, and also for stuff like voice-over-IP and online gaming.
It's rare to find a USB adapter for under $20, especially without a rebate involved, so this was a no-brainer for me. I haven't found any substantial reviews of the WN111, but I'm confident it'll do the job.
That said, based on some user comments I've read here and there, I'm going to download the latest drivers straight from Netgear rather than using the software that comes in the box. (Apparently the early drivers had some Vista compatibility issues.)
One other caveat: The 90-day warranty doesn't bother me, but Newegg has a no-refund policy on this product. That means you can return it for a replacement, but you can't get your money back if it doesn't work right or you don't like it. Not a deal-breaker for me, but I thought you should be aware.