Apologies if you're a regular reader of my TV Picks 'cuz you'll have seen this already.
You can now pre-order Season 1 box sets of the series Law & Order, Mad About You, Malcolm in the Middle, and Baretta.
DVD sets of the following series are in the works (I'll let you know when you can pre-order 'em): Magnum P.I., Battlestar Galactica, Dragnet, Earth: Final Conflict, Emergency, The Rockford Files, Land of the Lost, Quantum Leap, and Sliders.
You can now pre-order Season 6 of The X-Files on DVD.
You can pre-order the soundtrack to the "Once More, With Feeling" episode of Buffy The Vampire Slayer (a.k.a. Buffy: The Musical). This page has all the gory details on the album (it includes some extras, like a demo of one song as performed by Joss and his wife). It's being released on September 24th.
I'm trying to keep a complete list of what TV is available on DVD in the sidebar at TVPicks.net, in case you're curious what TV shows are available in the US on DVD (if you notice I've missed something, let me know).