Recs.
Updated
Text editor based on Scintilla (like Notepad++), Free Linux and Windows versions available, paid version for Mac.
SpecsUpdate
Pros
Pro Lightweight
With less than 2 MB of binary on Windows, SciTE starts instantly. Plus, if you don't need all the config, syntax files, blah, there's a 678k standalone .exe version. Nothing is going to beat that for lightweight and start-up times. Stick it in a folder that is already on your PATH.
Pro Powerful
Based on the Scintilla source code editor, SciTE has some advanced features like rectangular editing, simple regular expression search and replace, code folding, etc. It allows the user to launch a compiler or interpreter, and it can also interpret the error messages, jumping at the location they point to.
Lua scripting is key to SciTE's power and flexibility. The Lua scripting language can be used to perform complex text transformations. It's relatively simple syntax and its large user-base makes it a great choice for a scripting feature.