Many of the same in this thread, but no list is complete without Bonanza and the first three seasons of Dallas.
In no particular order... Top Ten Shows Still In Production Dirty Jobs Venture Brothers Lost Good Eats/Dining on Asphalt Daily Show/Colbert Report The Office Rescue Me Bleach (although the manga was better) Mythbusters Futurama (they're making movies!! ergo still in production) Top Ten Shows No Longer in Production Kids in the Hall TV Nation Homicide: Life on the Streets Touching Evil The West Wing DS9 Firefly Farscape Buffy/Angel (it's the same universe, it counts as one show!!) The Maxx