Free Applications

Try downloading some of these, after all you can't complain about the price. I've marked up most with their platform, and whether they're Open Source Software (OSS).

Antivirus / Anti-malware

MS Security Essentials (Win)
Lightweight free antivirus from Microsoft.
AVG Antivirus (Win)
A free virus scanner with free updates. Reasonably well regarded and popular, you should definitely consider installing this on any unprotected Windows machine.
Free-AV (Win)
I've never tried this, but it looks to be a worthy free alternative to AVG.
Avast Antivirus (Win)
Likewise, another free scanner I've seen in my travels.
Clamwin (OSS, Win)
An open source virus scanner based on the multiplatform ClamAV.
Spybot Search and Destroy (Win)
Hunts down spyware/adware and other nasties.
Ad-Aware (Win)
Another antispyware app.

Browsers

Mozilla Firefox (OSS, Win/Linux/Mac)
This is a browser-only Mozilla distribution with tabbed browsing support, popup-blocking, and a huge list of 'Extensions' that add immeasurable features. It's open source, cross platform compatible, fast to render, and a lot more standards compliant than IE6.
K-Meleon (OSS, Win)
This is a native Windows frontend for the Mozilla rendering engine, a similar concept to the Galeon and Epiphany browsers for Linux. Very fast to download and run.
Opera (Win/Mac/Linux/etc)
It has adverts, but since version 7.0 I've had to revise my opinion of this browser a lot.
Chromium (OSS, Win/Mac/Linux)
Google Chrome, without the Googley bits, if you care about privacy.
Evolt's browser list
Once upon a time I spent hours and hours looking for IE4.0 for 68k Macs. I finally found it, but my search would have been quicker had I headed to this first. If it can connect to a website, they have it listed, so if you're looking for a specific version of a browser hit that link. Also visit the rest of the site which has many good web-development related articles and links if you have the time.

Chat & Instant Messaging

Pidgin (OSS, Linux/Win)
A great open source multiprotocol messenger.
Exodus (OSS, Win)
An open-source client for the open-standards-based Jabber instant-messaging network. If you have a (free) Jabber account you can chat to other users and also use server-side 'transports' to add contacts from other services like MSN, ICQ etc. You can also establish encrypted connections to your Jabber server if you want. Worth a look if you're sick of bloated clients but still want to chat to many services.
Miranda IM (OSS, Win)
An open source and highly modular instant messaging client. It has 'plugins' to connect to almost every conceivable IM network on the client side, and customise its interface.

Editors

Notepad++ (OSS, Win)
A great free Windows editor packed with features and a usable UI.
Geany (OSS, Linux/Win/Mac)
A lightweight IDE with great code editing features.
SciTE (OSS, Win/Linux)
It is hard to set up, requiring you to edit a configuration file, but is blazingly fast and has good tab support and syntax highlighting for a huge number of languages.
Metapad (Win)
This is extremely fast and about 50k to download. I overwrite Notepad.exe with Metapad.exe on my system and haven't looked back. No syntax highlighting or tabs; just plain powerful text editing support.
WinMerge (OSS, Win)
Quickly spots the difference and merges two similar but different files.
Meld (OSS, Linux)
A file merge utility for Linux.

There's also VI and EMACS, but I'm steering well clear of that debate :). If you have heard of them, you probably already use one or the other.

Games

Soldat (Win)
OK, so it's not an application, and the latest versions are now technically shareware instead of freeware, but it's a great game nonetheless. Think of a cross between "Worms" and "Counter-Strike", with great multiplayer support, and a refreshingly simple story: 'A couple of guys decided to kill each other and you can be one of them'. Runs well on older computers.

Graphics

The GIMP (OSS, Win/Mac/Linux)
No, this isn't what you think. A Linux stalwart, the 'GNU Image Manipulation Program' has been ported over to Windows and Mac! It's worth it simply for the huge number of filters and effects offered, putting Paint Shop Pro to shame. The interface can be tricky, but it's highly reccommended.
Hugin (OSS, Win/Mac/Linux)
Free panorama stitching software.
Qtpfsgui (OSS, Win/Mac/Linux)
Free HDR compositor, for keen photographers!
IrfanView (Win)
Halfway between an application and utility, this is an excellent freeware graphics viewer. It supports operations like resize and filtering on images, and a massive variety of formats. Best of all, it's got neat 'batch' capabilities if you've got a few hundred images that need tweaking.

Office

LibreOffice (OSS, Win/Linux)
Formerly OpenOffice, this open-source is like Microsoft's Office, only free. It works with Office files, and pulls all the standard tricks like underlining misspelled words and so on. With text document, spreadsheet, database, graphs, formulas, and image support, it's very comprehensive and worth a look.
AbiWord (OSS, Linux/Win/Mac)
A lightweight yet full-features free word processor, great for older systems.
Gnumeric (OSS, Linux/Win/Mac)
A spreadsheet app that's light but very powerful.

Webservers

Apache (OSS, Win/Linux/Mac)
The standard. Yes, it requires some configuration, but setting this up on your home computer is a useful setup if you want to get into serious and/or CGI development.
Lighttpd (OSS, Linux/BSD)
A lighter alternative to Apache.
Nginx (OSS, Linux/BSD)
Another lighter alternative.
AnalogX SimpleServer (Win)
A 250k web server that you configure by dropping an HTML file into its window. A very quick and easy way of testing out sites 'online'. Has CGI support according to its feature list, but doesn't seem to run PHP/Perl/etc. well.

Web-based

Roundcube Webmail
Install nice Ajax webmail on your own server.
TT-RSS
A very powerful AJAX RSS reader
Laconica
Install a Twitter-alike on your own webserver.

Operating Systems

Ubuntu Linux
Or your favourite choice of distro ;). If you're read this far, you really ought to give Linux a shot on your computer!
OpenWRT
Linux on your Wifi router? Yup.

Download Directories

Free Software Downloads
A collection of multiplatform file downloads.

Tell Me More

If you have a few indispensible apps you use and recommend, please drop me a line.