It took some time to put this list together: I am sure it will help you to save a lot of time (and money) as it did with me!
OpenOffice Suite: text, spreadsheet, presentation, drawing and database. It's multi platform — learn once and do not loose time when you move to a different OS, saves documents also in PDF format and, last but not least, it is totally FREE;
SQLyog: a really powerful open source MySQL Manager and Administration Tool;
FireBug: all of the tools you need to poke, prod, and monitor your JavaScript, CSS, HTML and Ajax are brought together into one seamless experience, including a debugger, error console, command line, and a variety of fun inspectors.
FireFTP: do you really think you need to buy a software to do some serious FTP?
IE Tab: to see how your pages look in IE without the need to load it
PDF Download: the best way to handle PDF docs in your browser
Bookmark Sync and Sort: is a Mozilla Firefox extension that let you connect to an FTP/WebDAV server and synchronize your bookmarks that are stored in an XML file
Minimize to tray: minimizes Windows versions of Firefox, Thunderbird, Mozilla Suite, and Nvu to the system tray