coyote6
Adventurer
Transhuman Space - Lots of GURPS source books impress me, but this one impressed me the most.
Speaking of GURPS -- GURPS wowed me, itself. But I'll point out another pair of David Pulver's settings: Technomancer and Reign of Steel. I never quite played either one, but I ripped off lots from both, have read both a few times, and still want to run something in those settings one day.
The penguins in Technomancer are hilarious.