Whats your favorite power tool that helped you the most? OS doesn't matter.
Please note this is a community wiki. I would like this list to help all of us in finding more powerful tools.
If possible, list down one tool (or multiple tools for multiple os) per answer. This will make voting for tools easier
RapidEE for (finally!) painless environment variable management on Windows
So many to choose from...
Both count in my book for generality and availability.
I would have to say one of the following:
I am a programmer and power tools to me may not be power tools to you! I like writing my own when I need them as well.
On Linux (and many other Unix) systems strace. It allows me to debug system administration problems by looking at a command's interface to the operating system. It works on any binary (doesn't require compiling with debug symbols), and is especially useful if I don't (or can't be bothered to) have access to the command's source code. It can pinpoint configuration errors (where is this program looking for its input files?), uncover undocumented features (why is the program trying to open this file?), and performance problems (the command is writing to the file a character at a time). Similar tools are ltrace (Linux; works at the level of library calls), truss (FreeBSD and Solaris), and, of course, dtrace (Solaris, Mac OS X, and FreeBSD).
Bash. It does so much stuff well that would be cumbersome in a GUI.
UltaMon (for Windows) . I love being able to have my task bar stretch across all three monitors and easily move apps from one monitor to another without having to minimize and drag.
On the Macintosh it would have to be TextMate.
I can't imagine life without GNU screen. There are so many features that just make things a bit easier. Stuff like being able to detach a session and then attach to from any computer anywhere, with all programs still running. Or searching through the scroll history. Or running several screens in the same terminal window. And lots of other stuff.
Windows Key + R = Instant access to everything. In addition, I create batch files in the windows directory so I could do tasks that usually takes multiple commands with one command.
Dexpot + Clipx + Winsplit revolution + Input Director
Multiple desktops, clipboard history, fast & easy window positioning and using networked computer screens as simply as a multi-monitor setup.
And launchy, but has already been mentioned.
And Vim, of course.
Ditto is an extension to the standard windows clipboard. It saves each item placed on the clipboard allowing you access to any of those items at a later time. Ditto allows you to save any type of information that can be put on the clipboard, text, images, html, custom formats, .....
The Everything desktop search tool. It's face-meltingly good. Fast, flexible, unthinkably small (the complete download is 334Kb!)
The first thing I do on a new computer is install this and Autohotkey, and remap Win+F to run it.