WPide by WPsites replaces the default WordPress code editor for plugins and themes. Adding syntax highlighting, autocomplete of WordPress functions + PHP, line numbers. The plugin uses the Ajax.org Cloud9 Editor (http://ace.ajax.org/).
Autocomplete of WordPress functions and PHP
Automatic backup of every file you edit. (one daily backup and one hourly backup of each file stored in plugins/WPide/backups)
Highlight matching parentheses
This plugin does not currently work on Internet Explorer!
Download and info: http://wordpress.org/extend/plugins/wpide/