I'm surprised your DVD drives didn't come with software. I have copies of WinDVD and PowerDVD from my gf3 and fx5900 video cards, liteon and Asus DVD-ROMs, Pan A06 burner, Asus mobo, and Herc GTXP soundcard.
Also, Mwave and probably newegg sell the OEM versions for under $10 in their software section if you're buying the rare drive that doesn't bundle it.
I don't know of any free software though, since the DVD Overlords charge a royalty on decoder software. Even MS' media player must piggyback itself on whatever other decoder you've installed.