I don't know too many films he has been in apart from Hitcher and the terrible Hobo With A Shotgun (save yourself time and never watch it). I have recently picked up cheap The Legend of the Holy Drinker.
No doubt my eye will be drawn to some of his other work now.
Ladyhawke has always had a loyal following, and is definitely worth a look.
Personally; unless you haven't seen the TV series, I wouldn't bother with Buffy the Vampire Slayer. On it's own it's enjoyable fluff, but there's a reason Joss Whedon was so desperate to completely redo it. It does illustrate Hauer's unsurprising knack for playing vampires, but it's perhaps most notable now for giving Hilary Swank her first movie role (she plays one of Buffy's schoolmates).