Steven Slate Audio Trigger 2 Platinum

It's a great tool in the studio! I use it in every single project even if I'm not using drum samples.

Pros - it can create MIDI of your audio peaks (drum hits) so so much more accurate than the DAW's built-in Audio-to-MIDI function! Makes work much faster and easier to set up my MIDI for gates!
Also I like the way it handle samples - the built-in mixer.
I didn't really use it for real-time triggering only when recording triggered kick drum I using it to feed a kick sample sound to the drummer's headphones.

Cons - The "tune" funcion could come in handy sometimes but it's not that great - you better looking for other plugin (waves) for that.
If you have multiple samples you want to blend you need two audio tracks with 1-1 trigger instances on them because you cannot delay the samples inside the plugin's mixer to phase align them.

If you want to program drums from scratch you better look instead into other stuff like GGD - but if you want sample replace/reinforce drums or make your own samples this is a no brainer!