It's been a long time since I switched from using Windows as my primary development platform, and whilst you may hear a fair bit about the effect that this has had within my posts (for better or worse). Something that i have never really been able to completely replace on either the Mac or Linux platform was my favoured editor - Ultraedit. Deep down I have a soft spot for Uedit, and whilst it can be a little bloated for simple tasks, years of useage and tweaking to my liking made it my favourite tool for all kinds of development work.
Gedit on the Linux platform, once endowed with a few useful plugins, becomes a useable tool for web development. The inclusion of tools such as the 'Class Browser' (http://www.stambouliote.de/projects/gedit_plugins.html), 'Symbol Browser' (http://symbol-browser.sourceforge.net), 'Word Completion' (http://users.tkk.fi/~otsaloma/gedit) and 'Find in Files', makes Gedit a really useful editor, which whilst not really feature packed is more than capable, which is excellent for the standard packaged editor that is supplied with the Gnome desktop.
For the Mac, there are quite a few editors out there, some free some not, none are really on par with Ultraedit featurewise, but this could be seen as a plus in some respects. I was quite happy using TextMate for web development work, especially as it had plugins available for code browsing and project handling, that is, until I found Coda.
Coda, takes up where other editors on the Mac finish, and whilst it may only be aimed at web development, it includes everything that you might need in the day to day life of a web dev.
Coda integrates the most useful programs all into one single IDE, it includes project management, an FTP client, text editor, preview window, css editor, SVN client and even a terminal window for remote access to your web server.
Coda is not a free application, but does offer a free demo. If you're in the market for a decent development platform for web apps, then give Coda a try, I'm sure that you will not be dissapointed.
Coda is available from Panic software at http://www.panic.com/coda/