Komodo Edit — Free editor for Linux, Mac OS X and Windows
Komodo Edit is a free, multi-platform, multi-language editor for Web coding in (X)HTML, CSS, dynamic languages such as JavaScript, Perl, PHP, Python, Ruby, Tcl, & Ajax technologies. The list of features includes autocomplete and calltips, syntax checking and coloring, code snippets and folding, project management and collaboration aids — in short: everything you’d expect from a modern editor, especially considering the (nonexistent) price tag. Since Komodo is built on Mozilla technologies, it also supports XPI extensions, which provide a great platform for extensibility.
A commercial IDE is also available for $295, which includes support for CVS, Perforce and Subversion source code control plus various debugging tools, access to browser-server communication, and a few other goodies that help you “walk the DOM” or craft complex regular expressions. If you decide to spring for the full-blown IDE, the user-based license allows you to use the software on any of the supported platforms — a big plus if you work on Linux, Mac OS X and Windows — no nasty platform lock-in!