Katie Cook drew 200 sketch cards for an X-Men set and you can see 197 of them in that link. I had to share the Snowbird ones with you because, come on... it's Alpha Flight, but they're all just as awesome.
Operation: Black Gold
By Howard Purcell.
The Trouble with Mars
By John Stewart.
The Masters of the Pit
By Jim Steranko (courtesy of Golden Age Comic Book Stories).
Thongor: Warrior of Lost Lemuria
Also by Jim Steranko.