This one is easy. Got to be my U.S. Robotics Courier modem. It is the oldest component that I still have connected to my fairly modern system.
This was my second Courier. The first one was purchased in 1987 for the measly sum of $500, and served me well for a LONG time. I don't really remember what happened to it. It didn't die, but just disappeared. It may still be alive and kicking somewhere.
In 1993, I bought my second Courier, a 28.8Kbps model. Since then, it's been flashed to 33.6Kbps, to X2 56K, and to V.90 56K. What a workhorse of a modem. Eight years later, it still is more powerful than 90% of the analog modems out there.
Runner-up would be my ABIT BH6 motherboard. Three years strong, went from running a PII-350 to a PIII-900 with a simple BIOS flash. Pretty respectable for a 3-year old board.
Sure, I've got lots of older components lying around that are still alive (old video and scsi cards, etc.) but it's the older hardware whose performance rivals the newcomers that really impresses me...