Always admire folks who can pick just the right combination of fonts.
Unfortunately I'm not one of them.
I know the right combination when I see it, but ask me to come up with the solution and you would be sorely disappointed.

"What tools or services do you use and recommend for discovering complimentary web fonts?"
I tend to stick with the fonts and font combinations that come with the themes.
I figure that the designers are pretty good at it and that I'm not likely to improve things.

I see a lot of examples of themes that have been "customised" for fonts and colours and the results can sometimes be awful!