I just took some time to add (almost) all the keywords supported by lammps to a new language definition in Notepad++ (OSS). It's a free text editor but very powerful. Windows binary and source are available at http://notepad-plus.sourceforge.net/uk/download.php. You can use it on Linux as well if you know wine. (see: http://notepad-plus.sourceforge.net/uk/nppLinux.php)
To add highlighting and auto-completion support for lammps commands, please download the language definition file for Notepad++ at http://9rivers.linkka.com/2009/10/18/keywords-highlight-auto-completion-lammps-input-file-notepad/ and copy it to the folder mentioned in the post. Hope you find it useful!