emacs, git and a decent shell like bash with at least gnutools

emacs, because I was searching for the right editor for years
- multi-platform
- extensible
- ready to type (no fucking mode change for typing like vim)
- programming functions like auto indenting, syntax highlight, auto complete, etc.)
- multiple windows in any arrangement
Additionally
- it is completely programmable to do anything you want
- you can find a solution to most common development needs on the web

git, because
- it is usable from small personal projects to heavy duty development
- fast branching and checking out, switching between different workpaths within seconds
- basic version control offline, you only need to be online for remote consolidation
- you don't have to think much about structure from the beginning, if in doubt just commit and your work is saved, then arrange the result when you're ready

sh/bash-like shell with gnutools, because
- simple tools do their job and try not to be smarter than the user
- tools can be combined in any possible and impossible variants
- powerfull scripting (although sh-syntax is often annyoing)
- open as many shells as needed, no single-instance problem as with some GUI-tools
- extensible with gazillions of other tools

And best of all, all these tools are available on all widely used desktop OS.