Buy.com and Linksys are running some decent sales on the Linksys EtherFast 10/100 USB Network adapter. As far as I can tell, it should work well in Linux.
Here's some links, hopefully they won't change:
The Linksys ETHERFAST 10/100 USB NETWORK ADAPTER
If you check this page carefully, you will notice that Linksys is offering a $15 off mail-in coupon that you can print-out. You can also combine this rebate with other devices, if you are going network-crazy.
Also, this place has a link to a $10 coupon for buy.com.
Counting all the coupons and rebates, I was able to get this network adaptor and a 50' crossover cable for $23 postage-paid.