Help Wanted: Fiction with More Pulp

Throughout 2008, I tried to learn as much as possible about crime and mystery fiction. i started with hard-boiled, moved to noir, got a taste of the traditional, and even threw in some comedy. I am far from an expert but I at least know where to look.

I realized I still have a deficit: old-school pulp fiction. By pulp fiction, I'm talking not just westerns but adventure fiction. My recent reading of a Tarzan short story made me realize I was missing something. It's the stuff I probably should have read when I was in middle school (Tarzan, Mars books, Fu Manchu, Quartermain) but never did. Now, I'm planning on rectifying that deficit.

Patti Abbott's Friday Forgotten Books Project is helping (thanks August West for the G-8 reference). But I'd still like y'all's help in compiling some lists.

What are some great pulp adventure stories/authors?

What are some pulp western stories/authors?

What are some great pulp science fiction stories/authors?

What are some great pulp spy/espionage stories/authors?