My favorites so far have been 'cerulean seas', 'psionics unleashed' and 'strategists and tacticians'
Cerulean Seas because I love the idea of a completely underwater campaign
Psionics Unleashed for the obvious reasons
Strategists and Tacticians gives characters of all classes more varied options, and limb severing! I love that there is a prestige class that requires you to have replaced your limbs with prosthetics.
I was impressed with how much thought went into Cerulean Seas. For folks looking for underwater settings or rules, it's a good book.
I'll have to give Strategists and Tacticians a look. We had a prosthetic limb prc, but it doesn't translate well to pathfinder.