Everything search app is easily my most used on my Windows PC. Instant, as-you-type file search by name with detail sorting and wildcard support. I set a keyboard shortcut as well. It puts Explorer search to shame. As good as Spotlight, which I was missing from when I had a Mac in 2008.

LaunchyQT - modern fork of Launchy that actually gets development. App launcher, dead simple.

FanControl - Fan curve software with all the features I want including hybrid temp sources, sensible automatic settings, visual graphs with an intuitive interface.

AutoHotKey - Hotkey scripting language that I can use even as someone who barely codes.

Transcribe! - Not open-source but it's a reasonably priced audio and video slow down app for transcribing.