Linux Mint (both the Debian Edition and the one based on Debian via Ubuntu); I've been free of Microsoft products for awhile now (never had a Macintosh, but I used them in school) with no regrets.

I also appreciate Firefox, Thunderbird, Libby (by Overdrive, for accessing public library materials), Calibre (for epub management and sometimes reading, often for easily looking up cooking recipes thanks to full-text search), and KOReader (my child needs the text bigger than the stock Kobo can do; KOReader delivers, on this and other features). Oh and Steam, for making it easy to keep up with friends via videogames despite having ditched Windows.