Ubuntu - One of the easiest ways into the world of free operating systems. I highly recommend it to anyone bar people who are looking for an alternative to OSX and Windows. Also with Steam support, gaming on Linux is becoming more and more common place.
distrowatch.com - Although not a OS it's self, I use distro watch a lot as it usually links to the latest alpha / beta builds of the various OS distributions.
Gimp [Windows, Mac, Linux] - This is to Linux what PhotoShop is to PC. Coincidently, this is also for PC & Mac too!
Paint.net [Windows] - One of my favourites for use with a Wacom pen tablet.
Blender [Windows, Mac, Linux] - The freeware equivalent of Maya. I've seen some awesome projects done in 3D world magazine although I must admit to being primarily as Maya user as that is what I was taught at Escape Studios.
This ones a bit of a bugbear discussion of mine with people who refuse to use AV software as there are loads of good ones knocking about, and for completely for free!
Microsoft Security essentials [Windows]- It sounds basic and it kinda is but it works well!
Avast! Free [Windows, Mac, Linux] - It's free but it does make you register your details. I've used the paid product as well and thats pretty good also! I like the boot time scanning before it enters Windows.
FileZilla [Windows, Mac, Linux] - File transfer software. Pretty comprehensive. The server software is Windows only.
Telnet / SSH Client:
Putty [Windows]- A free Telnet / SSH client. Used for things like connecting to a remote shell on a Raspberry Pi
Notepad++ [Windows] - You'll never use MS notepad again. The syntax highlighting alone is immensely useful for various languages such as HTML and C++.
Atom.io [Windows, Mac, Linux] - I really like this git 'hackable' text based editor.
CPU-Z [Windows] - Tells you everything you need to know about that processor in your PC as well as more such as memory and timings. Dead useful for helping with upgrades.
HD Tune [Windows] - For testing storage drive speeds.
Firefox [Windows, Mac, Linux]- The first major competitor to Internet Explorers dominance!
Chrome [Windows, Mac, Linux]- My current choice of browser.
Thunderbird [Windows, Mac, Linux] - One of the best alternatives to MS Outlook.
Libre Office [Windows, Mac, Linux] - Newer than than the next but not any more advanced from what I can tell.
Open Office [Windows, Mac, Linux] - Still current but Libre office broke away. I can't tell them much apart to be honest.
GNS3 [Windows, Mac, Linux] - A free network simulator, you'll need some IOS images though for cicso equipment.
Wireshark [Windows, Mac, Linux]- Protocol analyser
Keyboard and mouse sharing:
Synergy [Windows, Mac, Linux] - like having a KVM in use simultaniously accross various PC's next to each other at the same time. It'll save you a ton of desk space only having one keyboard and mouse to rule them all!
Surprised at just how many are cross platform? I was at first but as I use nearly all three OS's daily its a massive convenience!
Also there is a rather large trend towards apps in 'the cloud' and I haven't touched on those useful apps such as Evernote, Lastpass and so on. I will save that for another day.
Feel free to post practical suggestions in the comments.