Picked up a Linksys WRTP54G at Goodwill tonight for $20. It was in the box and even in plastic, but had clearly been used before. It had a WEP key configured for wireless. It's also missing the antenna, but I don't plan to use it for wireless anyway.
Unlike the Vonage box I got a few months ago and tried fruitlessly to open up, I actually found a procedure that worked for this device. Here it is for future reference: