Gnome Wave Cleaner

If you are working on a project transferring analog tape or records to your computer, you may have found the need to clean them up a bit. Maybe they need noise reduction or click removal? I've tried various software for this and for noise reduction in particular, I have found the Gnome Wave Cleaner to be the best by far. Even though I use Audacity for most of my projects, this application gives you much more control over noise reduction - helping to eliminate the "metallic" sound that sometimes comes from the process. Highly recommended!

Sweep

According to the website, "Sweep is an audio editor and live playback tool for GNU/Linux, BSD and compatible systems." I haven't tried it yet so I'm not sure what special features it has when using it as a "playback tool" but sounds interesting! For audio editing it seems like it would be a good alternative to Audacity if for some reason you need or want one. You can get the application, tutorials and more here.

CamStudio

CamStudio is an open source screen recorder for Windows.
It can record the audio and video of your screen (or a selection of the screen, like a window) to an AVI file. It is very useful for creating tutorials.

Seq 24

Seq 24 is a midi loop sequencer for Linux and Windows. It has a very simple interface and works well when I have used it under Linux, using Jack to connect it to various midi soft synths.
It appears that it hasn't been updated for some time but the latest version worked very well for me.

HandBrake

HandBrake is an open source, cross platform application that can convert a DVD to mp4 files. Perhaps not something that you need every day but when you do... here's a great application to do it with!