I bought a TI-86 graphing calculator for $10 new in 2004. That was two years before it was discontinued, so that was an awesome price. I absolutely love that calculator. It's so easy to program for, it has custom shortcut keys, fantastic battery life, and has a super convenient multi-unit converter. The Casio FX-9750 GIII has a similar conversion feature, but it requires like, two extra button presses per conversion. I still use this calculator all the time.
Programmable gaming mouse. I have so many key combos and macros on my mouse, I could do most of my job one handed now if I had to. It's been so helpful for things like holding the baby when I'm working and my wife is unavailable.
Steelcase Leap V2. I can put in a 70 hour work week with no back pain now.
A fountain pen. Doesn't even have to be an expensive one. Some of my best memories can be tied to cheap pens. Signing for my house with my Kaweco Sport. Or signing my marriage license with my Jinhao x250. I feel like I just appreciate what I'm writing more with a fountain pen.
My PS2. Well, it was a gift. But still, I've had it since launch and it's only gotten better with time. Free MCBoot, hard drive, Noctua fan mod, Brooks PS3 controller adapter.. It's basically the only console I play anymore.
Giving up and just buying a gaming mouse for all my mouse needs was probably number six on my best life decisions. I'm prety much in a committed relationship with the Razer Basilisk X HyperSpeed model and I give it as gifts to friends who have no idea that's what is missing from their life.
Very nice! I'm using the Rosewill Neon M63 right now. With all the RGB turned off, it's just a plain looking black mouse with a bunch of buttons on the side. Looks like it's half the price I bought it for now, I should get one for work so I don't need to keep bringing mine back and forth when I'm in the office. It's pretty great to have common NX and SAP commands ready to go.