At any given time, regardless of which machine I'm on, I typically have these guys open:
OS X Mail - 4 Mail Accounts
(Usually) iTunes (Sometimes) Spotify
Anyone that's wondering, Tweetbot is worth the $20.
f.lux - Adjusts your screen's colour temperature to match your room's. Changes automatically throughout the day as it gets dark. https://justgetflux.com/
Keka - 7Zip file archiver. Compresses up to half the size of standard zip files. http://www.kekaosx.com/en/