As a Ruby developer who typically works on MacVim, pretty much everything I use (except OS X and it's built-in apps: mail, preview, itunes, etc.) is open source. I have other things like Omnigraffle, The Hit List, and some games, but those are used much less frequently.
95% of my day I'm looking at MacVim, iTerm2, and Chrome, which is essentially open source as well since it's mostly Chromium.
Servers are Ubuntu running Apache, Passenger, MRI Ruby, Memcached, CouchDB, ElasticSearch, Redis, PostgreSQL.