RealMedia Helix Producer Basic-- lets you create realmedia files. The free version is fine; the features that are omitted may not be that important for occasional use.
FFMpeg -- an open source compressor that converts from one file format to another. In it's native form, it's a command line tool, but many programmers have built tools that use a graphical interface with it. Check out all the projects that use it on it's home project page -- many of these tools may be available not only for Linux, but also for Windows and Mac OS X.
MPEG Streamclip -- Essential, free tool for cross encoding video, particularly useful for turning video in mpeg-4, or taking unprotected VOB's from DVD's and turning them into editable DV. Mac OS X and Windows XP.
FFMPegX -- A swiss army knife for video recompression, but requires a bit of time to learn to use. It adds a busy but useful GUI interface to FFMpeg. It can cross encode to and from several formats, most importantly mpeg 1, 2 and 4.
Iriverter -- Open source cross platform compressor for mpeg-4 content, particularly portable devices such as iPod Video, PSP, and some of iRiver's players.
MakeRefMovie -- It is designed for creating reference movies that can point to other movies, but it is a very useful tool for creating movies that point to streaming Quicktime content. For both Windows and Mac.
Windows Media Encoder -- A fairly powerful and complete solution for compressing avi files into streaming wmv content. It can even be used a a screen capture tool. It's free, but only for Windows.
VirtualDub -- an extremely handy application for the Windows platform for capturing, manipulating and compressing video. Well worth having.
Still Life -- one of several applications available that allow you to pan and zoom with still images, an effect that has been used in filmmaking for a long time. It's cheap, has a very intuitive interface, supports slick features like bezier curve pans, tilt, spin, and can even export the finished file as an iMovie project.
Pan Canvas Pro -- Another application that allows you to pan and zoom in still images. One particularly notable feature (other than it's clean interface) is that it supports ease-in ease-out (deacceleration). Avaliable for be both Windows and the Mac.