Lulu and IPR for some stuff. Ronin Arts sells stuff through Lulu as well as a couple of other places.
Your Games Now I've just started to use.
As for how pdfs have helped my game...
Well, being able to get electronic copies of books that are otherwise difficult to get hold of is one nice plus for me. For example, the Artificer's Handbook from Mystic Eye Games. The D&D Rules Cyclopedia is also in pdf form (saves wear and tear on my HB)
Being able to take individual pages and print them for handouts is pretty nice.
Being able to search the pdf for a particular phrase when you're not certain where it is in the book, or just to find all the references to a particular thing.
The ability to print the pdf and not have to worry about the printed copy get trashed, unlike your nice book. Sure the cost of a self print or print-and-bind from someplace like PrintFu might be close to replacing the book that you'd have bought in the store, but it doesn't really hurt in the same way.
I like the fact that I can have a large gaming collection that weighs ounces instead of pounds. Portability is nice.
Also, I tend to use things like Treepad Light to organize my notes. Being able to pop open a pdf and reference something "on the fly" as it were, is pretty nice. Especially since most of my books are kept in the bedroom, and I don't have to worry about waking my wife if I'm doing some late night stuff.
One thing that's kinda spiffy but really underused, is the ability to update the material. Not a lot of people seem to bother with updating their pdfs, and instead people have to poke around on forums gathering together scraps, and so forth.