For discipline specific applicaitons, click here.
Graphics
GIMPshop is a modification of the free/open source GNU Image Manipulation Program (GIMP), intended to replicate the feel of Adobe Photoshop. Its primary purpose is to make users of Photoshop feel comfortable using GIMP. In other words, GIMPshop is Photoshop, but without the cost.
ResizeIt allows you to change the size of multiple images simultaneously and very quickly. It can also convert file formats at the same time. It is suitable to make images to upload the homepage or to attach the email.
Seashore is a small and easy to use application that lets you add text or other simple graphics to images. Some like to use this to indicate what students should be focusing on in an image or to provide captions without having to insert the image into a document. But, it does more that than, even. Think of it as a Photoshop/GIMPshop lite. I even found some tutorials for Seashore that can be accessed here.
Productivity
Concept Mapping Tools
XMind is an open source brainstorming and mind mapping software tool developed by XMind Ltd. It helps people to capture ideas, organize to various charts, and share them for collaboration. It supports mind maps, fishbone diagrams, tree diagrams, org-charts, logic charts, and spreadsheets. It is often used for knowledge management, meeting minutes, and task management.
Evernote is an information organization tool. With the free version, you can save simple notes, images and even snapshots of web-pages in a key-word searchable database. Evernote even recognizes text in images that you enter, and makes that searchable. Evernote stores a local version on your computer, well actually as many computers or handheld devices as you want, and syncronizes them through an online database. The free account allows a limited amount of file transfers/month. To go to an unlimited amount a subscription costs $45/year.
Audio
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.