I love cross platform software because it allows me to move from OS to OS without having to relearn applications. In order to be considered for this list the software must run on (at least) Windows, Linux and Macintosh. In some cases I may bend the rules but probably not. 
See also: ApplicationSwitchMatrix, AppleSoftware, LinuxSoftware, WindowsSoftware
- 7-Zip (Open Source)
A file archiver which supports most formats including 7z, ZIP, GZIP, BZIP2 and TAR (and for unpacking only supports RAR, CAB, ISO, ARJ, LZH, CHM, Z, CPIO, RPM, DEB and NSIS). It is primarily a Windows tool but has been ported to Linux (it's in Debian) and OSX so it counts.
- Audacity (Open Source)
- 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.
- Azureus (Open Source)
A powerful BitTorrent client. I don't personally like it's user interface very much (it's seems unnecessarily complicated) but it is exceedingly capable.
- BlogBridge (Open Source)
Feed (RSS and ATOM) reader and aggregator. It also supports "BlogBridge Service Accounts" which allow you to keep multi copies in sync via a web service (it's free I believe).
- ClamAV (Open Source)
A powerful anti-virus tool with a community maintained signatures database. It's primary short coming is that it doesn't support on demand scanning (ie. when you open or write a file). I've been using it as a personal and enterprise mail server anti-virus solution for several years and in that time it's never generated a false positive and has only let two viruses through (which is a significantly better sucess rate then either of the commercial products I'd used previously). On the Mac there is a port called ClamXAV.
- FFmpeg (Open Source)
A set of CommandLine tools for video transcoding. Not the easiest to use but very powerful (many GUI applications are based on it).
- Firefox (Open Source)
The open source web browser.
- GAIM (Open Source)
Instant messaging program which supports AIM, Jabber, Yahoo!, MSN, IRC and most of the others. Technically it does run on Mac but you'd be crazy to, instead run the port called Adium X.
- Gantt Project (Open Source)
- Java based project management tool. Supports import/export of MS Project files and colloboration for multiple clients via a WebDAV server.
- GIMP (Open Source)
GIMP is an image manipulation tool which can do most of the things that Adobe Photoshop can. One of the complaints that many adopters have is that the user interface is quite different from Photoshop. To address this there is a port of GIMP made to work as similarly as possible called GimpShop. On the Mac there is a native port called Seashore.
Comment from BrettShand: in the end, IMO, the Gimp workflow is better than PhotoShop. It pays to learn the counter-intuitive Gimp interface.
- Gizmo Project (Freeware)
- A softphone and instant messaging client which supports standards based SIP. It allows free calls between the softphones and free calls between landlines connected to accounts (with some restrictions). Because it supports SIP it also works with normal VOIP phones and adapters.
- Inkscape (Open Source)
- Inkscape is an vector graphics editor, with capabilities similar to Illustrator and Freehand and is based on the Scalable Vector Graphics (SVG) file format. Supported SVG features include shapes, paths, text, markers, clones, alpha blending, transforms, gradients, patterns, and grouping. Inkscape also supports Creative Commons meta-data, node editing, layers, complex path operations, bitmap tracing, text-on-path, flowed text, direct XML editing, and more. It imports formats such as JPEG, PNG, TIFF, and others and exports PNG as well as multiple vector-based formats.
- jUploadr (Open Source)
- A cross-site Photo uploader which supports both Flickr and Zooomr. It allows you to set all properties of a photo before you upload it. It also supports batch editing, so you can make short work of uploading a bunch of files. It doesn't support Gallery but does have a plugin API/architecture so that support could be provided.
- KeePassX (Open Source)
- KeePassX saves many different information e.g. user names, passwords, urls, attachemts and comments in one single database. For a better management user-defined titles and icons can be specified for each single entry. Furthermore the entries are sorted in groups, which are customizable as well. The integrated search function allows to search in a single group or the complete database. KeePassX offers a little utility for secure password generation. The password generator is very customizable, fast and easy to use. Especially someone who generates passwords frequently will appreciate this feature.
- Mplayer (Open Source)
- A video player which will play almost any movie. It can be a bit difficult to get installed and working (which is why I generally use VLC) but it is an incredibly versatile tool.
- Open Office (Open Source)
- After a slow start Open Office is now a fairly complete replacement of Microsoft Office. Unless you are a power user there's a pretty good chance you won't even notice the difference.
- PowerFolder (Open Source)
PowerFolder is a tool to synchronize files between PCs in home-network, intranet or over the internet. On the internet PowerFolder utilizes peer-to-peer technology to discover the people you exchange files with. Main use cases for PowerFolder are the automatic file synchronization between PCs, private or public filesharing, the exchange of project files and data backup. PowerFolder is very easy to setup and has an intuitive user interface. Files are managed in virtual folders - multiple users or PCs may have access to the same folder.
- Scribus (Open Source)
- Scribus brings award-winning professional page layout to Linux/Unix, MacOS X, OS/2 and Windows desktops with a combination of "press-ready" output and new approaches to page layout. Underneath the modern and user friendly interface, Scribus supports professional publishing features, such as CMYK color, separations, ICC color management and versatile PDF creation.
- Skype (Free)
- A softphone which uses the Skype network to route calls to different people. Calls to other computers are free but bridging to/from traditional phone lines costs money (though it's cheap).
- SongBird (Open Source)
- Based on the Mozilla toolkit it's aiming to be a full iTunes (and better?) replacement.
- Thunderbird (Open Source)
- Not as good as Firefox but a pretty decent mail client. I'm continuously annoyed by small problems but have yet to find something I like better on Windows or Linux.
- Unison (Open Source)
- Unison is a file-synchronization tool for Unix and Windows. It allows two replicas of a collection of files and directories to be stored on different hosts (or different disks on the same host), modified separately, and then brought up to date by propagating the changes in each replica to the other. For those familiar with rsync Unision is basically the same thing only it supports bi-directional syncing.
- VLC (Open Source)
- A video player which will play most movies.
- WorkPace (Commercial)
A powerful piece of MicroPauseSoftware which runs on almost everything. It's not cheap but it works very well.
- Workrave (Open Source)
A nice piece of MicroPauseSoftware. It doesn't run on the Mac but I'm listing it anyway because it is the best free alternative (use AntiRSI on the Mac).
- Xara Xtreme (Commercial & Open Source)
- Xara is one of the best and fastest vector drawing programs available. Traditionally a Windows only tools but recently they decided to Open Source the code for Linux and Mac. The Linux port is nearly ready with the Mac port lagging somewhat.