- Babylon 5 - lasted 5 seasons, led to Crusade (and arguably Legend of the Rangers).
- Buffy - lasted 7 seasons, led to Angel.
- Hercules - lasted 6 seasons, led to Xena.
- Highlander - was itself a spin-off from the film, lasted 6 seasons, led to animated series and The Raven.
- Stargate SG-1 - was itself a spin-off from the film, has lasted 9 seasons and counting, led to Stargate Infinity and Stargate Atlantis.
- Star Trek TNG - was itself a spin-off from TOS/movies, lasted 7 seasons, led to DS9, Voyager, and Enterprise.
Charmed is the only other example I can think of (now up to season 8), but it hasn't finished yet, so there's still time. Quantum Leap is the next runner up (only lasted for 5 seasons).
I seem to recall that sulkyblue did a similar list of TV shows a while back (in the context of whether they made it to year 7), but I can't find it now.