Firefox
The open-source, free, standards compliant web browser. Have a look at my Recommended Firefox Plugins for some useful addons.

Notepad++
Notepad++ is a free source code editor and Notepad replacement.

Editpad Pro (not free)
Advanced Text Editor which recognises most popular file formats and is able to syntax colour PHP, HTML, PLSQL, CSS etc

Notepad++
Advanced Text Editor (available from PortableApps)

MP3Tag
MP3Tag is a powerful and yet beautifully simple tool to edit metadata of common audio formats where it supports ID3v1, ID3v2.3, ID3v2.4, iTunes MP4, WMA, Vorbis Comments and APE Tags.

Treesize Free
Treesize Free shows you how the space has been used within a collection of directories.

FileZilla Portable
FTP manager (available from PortableApps)

Oracle SQL Developer
Oracle SQL Developer is a free and fully supported graphical tool for database development. With SQL Developer, you can browse database objects, run SQL statements and SQL scripts, and edit and debug PL/SQL statements.

The Gimp
The free, open source image manipulation program with capabilities similar to Photoshop and Paintshop Pro.

Inkscape
An Open Source vector graphics editor, with capabilities similar to Illustrator, CorelDraw, or Xara X, using the W3C standard Scalable Vector Graphics (SVG) file format.

CDBurnerXP
CDBurnerXP is a free application to burn CDs and DVDs, including Blu-Ray and HD-DVDs. It also includes the feature to burn and create ISOs, as well as a multilanguage interface. Everyone, even companies, can use it for free.