Internet Explorer for Linux
For anyone developing for the web, cross-browser compatibility is (or at least it should be) important. Developing from a Linux environment usually necessitates having a copy of Windows and Internet Explorer hanging around for testing, either through running them on a separate computer or via an irritating dual-boot.
The IEs 4 Linux project provides a simple way to install Internet Explorer versions 5, 5.5 and 6 on Linux (using wine, removing the need for any of the above (although in order to comply with IE's licence terms, you'll need a valid Windows licence). Installation is by simply running a single executable and following the instructions.