I'm a big fan of "trick" monsters.
Over at Dungeon of Signs, Gus L did a nice list of special abilities that make a monster more than a straight-up fight:
Towards a Taxonomy of 'Trick' Monsters
Some of the abilities I wouldn't personally cal tricks so much as just special attacks - breath weapons are scary, for example, but don't feel like a sneaky trick to me any more than a bow and arrows is a trick weapon. Same with, say, a bear hug from an owlbear - it's just a jumped up melee follow. But it's a great list of special abilities, whether they are tricks or not.
The list is very D&D-centric, so a lot of the definitions aren't terribly generic (breath weapons as a HP-based attack, say), but even for GURPS it's a solid read and a useful resource.