App Launcher/Keyboard Interface

Quicksilver - An extensive tools that integrates into your system, from it's simplest level of an application launcher, using the keyboard interface Quicksilver can be so much more.


Back-Up Utilities

SuperDuper - Back up and clone your drives with this great tool

iBackup - A simple backup and restore utility, supports scheduled backups of files, folders and applications.


Browsers

Firefox - A top browser, now going into version 3 which is reportedly faster and a better performer all round - my personal browser of choice.

Opera - A most worthy browser that is to a lot of peoples liking, and have to admit i like most of it's features too.


Communication

Adium - The best multi account instant messenger including jabber and gtalk amongst the regulars along with many more.


FTP

Cyberduck - The best and easiest in use of ftp clients for osx.


Maintenance

AppDelete - Saves you going around deleting everything manually.

IceClean - A powerful general maintenance application bringing you the power of UNIX - akin to the likes of TechTool, a worthy addition to your arsenal it WILL come in handy.

MainMenu - Handy menu bar maintenance app to save you the time and effort.


MS Office Alternative

NeoOffice and OpenOffice - Essentially the same suite, this is your chance to say goodbye to MS Office and save yourself the money and hassle - NeoOffice is a little better at the moment as OpenOffice is just becoming OSX native.


Network Tools

Bwanadik - I only recently discovered this one, menubar app creating a shortcut to most of your network preferences, most notably displays your connections and external/local IP addresses.

iStumbler - Wireless discovery app, find those wi-fi hotspots, bluetooth devices and bonjour services.


Photography

JetPhoto Studio - This is one awesome app, photo library, image correction, flickr uploader and photo web gallery creater all in one - option to upgrade to Pro version.


RSS Readers

Vienna - RSS reader with some nice features, including growl integration, menubar icon, tabbed viewing of articles/subscriptions along with a few others.


Miscellaneous Utilities

Calq - On demand calculator, activate with keypress do your calculation and then it hides until needed again.

Desktop Manager - If you have machines that are not compatible with Leopard and you want virtual desktops, then look no further, the site says it is alpha quality but it works perfectly well and I use it on 10.4.11 - still going strong here :)

Growl - System notifications preference pane add on, plugs into an array of applications and system areas letting you know what is happening - including firefox, quicksilver, adium, mainmenu and vienna.

MagiCal - Handy menubar calendar and clock app.

Password Gorilla - Registrations and logins manager.

PTHPasteboard - Useful Utility that keeps a record of your pasteboard handy.