- Inkscape instead of Illustrator - GIMP instead of Photoshop (just install Photoshop keyboard shortcuts and use single window mode and it's much less "alien") - Iceweasel/Firefox instead of Chrome (super fast, works well with HiDPI and the dev tools are really on par now) - Steam (obvious) - pavucontrol as audio mixer - ncdu to track down disk space usage - sxiv to view images - zathura to view PDFs
In general I do a lot more stuff via the commandline now than before and a selection of custom scripts in ~/bin is growing. For example I have a script "serve" that will copy a file via scp to my webserver and return the URL that I can then share, and I have cronjobs that fetch my bank account balances and synchronize my mail via offlineimap etc.
And the thing I like most is that I know exactly what the running processes/services are doing; there are no adware/crapware/updaters; memory usage is minimal (about ~150 MB for a clean desktop); the system never swaps (8GB RAM).