A bunch of 1L glass bottles with wide openings so that they're easy to clean in the dishwasher - has reduced the friction to getting enough water in, and I can't remember the last time I felt that type of tiredness that comes from dehydration.
BenQ ScreenBar Plus - It's just a light that goes on top of your monitor, but it makes working much more pleasant. There's probably a clone that's cheaper and does the same thing, but I'd buy this one again without hesitation.
About $200 worth of home workout stuff (exercise mat, adjustable dumbbells, resistance bands, pullup bar) - I had to stop running for a while due to some injuries and I hate going to the gym, so this has been a game changer in terms of staying fit.