[this list is only in order as they come to mind, not importance]
1. Debian / Linux Mint : Since the new gnome came out, I transitioned to Mint for it's Cinnamon DE. It's an extremely smooth environment to work AND play in. Being Debian based, it's simple to keep a similar environment to production
2. tmux: The most bad ass terminal emulator on the planet. I couldn't imagine my workflow without it.
3. Firebug: Completely essential if you build any front-end
4. Git(hub): Versioning FTW
5. Dropbox: Helps me keep docs, scripts, dotfiles, graphics, etc sync'd across all my workstations
6. phpStorm: I used to rag on heavy IDEs. Now, I couldn't do without it
7. Bluefish: My favorite light-weight IDE. It works with SFTP so I can mount a remote server and make quick changes with all the colorization, syntax hilights, and code suggestions one needs.
8. GIMP: I often find the need to make quick changes to a graphic, or to grab a layer from a PSD.
9. LESS: A language, yes, but moreso an important tool for keeping organized CSS and compiling out minified CSS
10. Nano: Once you know how to use it, nothing else matters.