1. Buffy the Animated Series
This series was actually green-lit by 20th Century Fox back in 2002 (a year before "Buffy" went off the air), and most of the actors from the series -- save for Sarah Michelle Gellar -- were on board, but the production company couldn't get any network to commit to it, so the project died. The series would have been set in Season 1 (though in a continuity in which Dawn existed), and probably would have consisted of non-canon "demon of the week" style episodes, with a lighter tone than the live-action series. Every so often, rumors pop up about it coming back to life (like a vampire rising from the grave), but it seems the closest we're ever going to get to it is Issue #20 of the Buffy Season 8 comic (which used the style of the animated characters and was written by BtAS writer Jeph Loeb).
2. Faith the Vampire Slayer
With "Buffy" coming to a close in 2003, producer Tim Minear pitched Eliza Dushku on a potential "Faith" series, which would see the rogue slayer still living on the run from her imprisonment, traveling the world doing slayer-type things, helping people and discovering herself. Honestly, that sounds pretty awesome, but Dushku passed, choosing to do "Tru Calling" instead. Given that "Tru Calling" was canceled early in its second season, that might not have been the best choice, but I can't begrudge Dushku for wanting to move on from a character she'd already played on-and-off for five years.
There have also, at times, been rumors of a "Ripper" series or TV movie and a Spike TV movie, but it woudl seem like the time for both of those has passed. And now with the proposed "Buffy" movie reboot being dead in the water, it looks like we'll just have to survive on our DVDs for a few more years.