Mine - an Axe-Fx Mark II.
-DG

Easy: my patch panels. When you have a bunch of outboard patch panels make life soooooo much easier. 
 
 
Dean

New eye glasses on a new prescription.
 
Bob Bone
 

wireless keyboard and mouse. Such a "duh, this only cost $30, why didn't I do it years ago?" moment. 
 

Proper studio desk and rack mount cases.

Rme ucx was my latest hardware purchase and it has opened many new doors.

I 

gswitz
Rme ucx was my latest hardware purchase and it has opened many new doors.

What new doors did that open for you? 
 
Just curious....................

The ARC 2 system for mixing has improved my life.  It comes with a calibration mic, an omnidirectional condensor mic (which can be used for other purposes), is the hardware part of the package.  The room correction software for my monitors is what makes this package special.  Its virtual monitoring is worth the price by itself.

Modded my MOTU Core24IO with new low slew-rate ON-Brand OP-AMPS and added a BLA clock with an Apogee BNC cable. The OP-AMPS sound more open, and the clock improved transients on drums. I've also recently added a True Precision 8 Mic-pre. It's really tight, but HOT!