OpenSSL

The best OpenSSL introduction and tutorial is the book Network Security with OpenSSL, by Pravir Chandra, Matt Messier, John Viega.

So far the best online documentation I've found is An Introduction to OpenSSL, written by Holt Sorenson and published at Security Focus.

And of course there is the online manual pages...

Update 20070315
Found another article focusing on using SSL/TLS. An Introduction to OpenSSL Programming, part 1 and part 2 by Eric Rescorla hosted at Linux Journal.