Not so much software as an element that needs to be part of so much software but, for some unfathomable reason, is not:
Picklists which work properly. If I type “U”, do not give me a screen full of Ts with the first U at the bottom. And let me type more than one letter to get to exactly where I need to be in the list. It can be done but it rarely is done and it does my head in.
Also, if you must include a scroll wheel to enter numbers, make it possible to just fucking type the numbers instead.
This is probably not what you intended this thread for but I’ll take any chance I get to ask UI designers to get a fucking grip…