Here are some of the things I've bought and my quick thoughts on each:
Apple Logic 9 + Mainstage
Probably the most essential programs I have. I use GarageBand a lot, too, and Logic can import GarageBand files, which is great. Mainstage is great for playing guitar through.
Native Guitar Rig 5
Well worth the money (esp since I got it for free w/ my Native Komplete interface), but I find myself using Apple's built-in amp modelers a lot more lately.
Guitar Pro 6
We're all busy people, right? So I occasionally write parts for my bandmates to play and I can tab them out quickly in Guitar Pro, then make a quick playthrough video, and then they can learn parts at their own leisure and you don't have to waste time in rehearsal. If you're a writer, I think this program is essential.
This includes Alloy, Ozone, and Nectar. It's great for when I don't want to sit there and work out every effect I want to use on everything, I can have pretty powerful channel strips at my disposal.
Yeah, judge me all you want. This program is amazing. A little goes a long way, and it seems to work better than Autotune.
XLN Addictive Drums
Having tried other drum programs like BFD or DFH, this is my favorite. Also very affordable. The Ludwig kits are amazing. I will probably buy Addictive Keys soon. I have a drummer now but I still like to use Addictive Drums for demos, and I've also been pretty successful with sample replacement using Addictive.