Magic Bullet Looks and NewBlueFX (Motion Blends - transition pack). I don't need to say anything more about Magic Bullet Looks, everyone pretty much knows by now that's a solid product.
NewBlueFX sells a lot of plugins for effects and transitions. They are all pretty decent quality and very affordable but the Motion Blends are the only ones that I have. They're a pretty good step up from the PPro stock transitions and highly customizable. I find that when I need more pizazz in the transition, these do the job quite well.
Used to have the Trapcode plugins (Shine, 3D Stroke, Starglow) and found lots of uses for those over the years but I never upgraded them to the new 64-bit versions when CS5 came out so those are pretty much dead on my system. I've found that After Effects still lets me do a lot of the same stuff with the built-in effects there.