Here is a list of free, zero cost software tools that I use in various multimedia projects.

This includes creating content for websites, applications and video productions, managing my music collection as well as text and video editing.

General
Notepad++
Notepad++ is a free text and source code editor and Notepad replacement that supports several languages.

Graphics
The Gimp
The free, open source image manipulation program with capabilities similar to Photoshop and Paintshop Pro. The most obvious difference being the price.

OpenOffice Draw
A free, open source vector graphics application that for most people is a far better and cheaper option than CorelDraw.

XnView
View and convert graphic, photo and image files. It supports more than 400 graphics formats. It can read and edit EXIF data stored in digital camera images. I use it to bulk rotate photos based on their EXIF data.

Video
VirtualDub
VirtualDub is a video capture/processing utility for Windows platforms. It has batch-processing capabilities for processing large numbers of files and can be extended with third-party video filters. VirtualDub is mainly geared toward processing AVI files. Brilliant for slicing and trimming video files.

VideoLAN (VLC)
VLC is a free and open source cross-platform multimedia player and framework, that plays most multimedias files as well as DVD, Audio CD, VCD, and various streaming protocols.

Audio
Audacity
Audacity is free, open source software for recording and editing sounds. It is available for Mac OS X, Microsoft Windows, GNU/Linux, and other operating systems.

MP3Tag
Mp3tag is a powerful and yet easy-to-use tool to edit metadata of common audio formats and tags. It can rename files based on the tag information, replace characters or words in tags and filenames, import/export tag information, create playlists and more. Mp3tag supports online database lookups allowing you to automatically gather proper tags and cover art for your music library.

Myna Audio Editor
Use Myna to remix music tracks and audio clips. Apply sound effects and record your own voice or instruments!

Roc Music Creator
Use Aviary's music creator to simulate dozens of musical instruments including piano, guitars and drums. Create music loops and patterns for use in Aviary's audio editor (Myna) or as ring tones.