Oldest GPU you have and still working (Integrated graphics included)?
Radeon HD 7750 in a pure video streaming pc.
It does it's job (720/1k video 60fps) and has never acted up can't complain.
ATI Radeon HD 5570, my parents still use it in my old PC :)
ASUS EN9500GT MAGIC 512MB
2x Voodoo2 (12MB) in SLI running on an old Win 98SE box that I use occationally to play old games like High Octane on a CRT.
An XFX GeForce FX-5200 128MB 64 bits. Came on my first PC back in 2003, which I still have since I can't get the courage to throw it away (too much nostalgia). Paired with a P4 (1.8GHz) and 1GB of 333MHz RAM. Not really a fast PC, but I had a lot of fun playing the 3D GTA trilogy, NFS: Hot Pursuit 2, NFS: Underground 1 and 2, Diablo II, Age of Empires III, Age of Mitology, Call of Duty I and II, Medal of Honor: Allied Assault, Half-Life 1 and 2, Counter Strike and a few others. Most of them on Low at 800x600 on my 17" LG Studioworks monitor, but still, lots of fun.
Note: Buying a 64bits FX-5200 (or any FX-5200 to be honest) was the equivalent of buying a DDR4 GT1030 nowadays.
Might have a ti4200 in a box somewhere too but I don't know if it works.
Chaintech FX 5200
Does my Intel G41 Express Chipset count ? :P
The 2008 iMac I'm using right now has an ATI Radeon HD 2600 Pro 256 MB.
It's fine, because all I use my iMac for is writing and a bit of web surfing.