I wanted to start a list of free and open source software, particularly those pertaining to any of the various aspects of developing flash games: writing, programming, art and music. The following is a list of programs that I have used or have passing familiarity with.

Word Processing
Open Office

Vector Graphics
Inkscape

Image Editing
GIMP
MyPaint

2D Animation
Pencil(in beta)
Synfig
KTooN

3D Animation
Blender

Audio Editing
Audacity

Programming
FlashDevelop

Flash AS Libraries
Fixel
Box2D

Game Engines
Panda3D(not Flash, but cool nonetheless)

I am sure there are many more free/open source programs out there that I have not stumbled across. Feel free to add to this list with the name of the software, its category and a link. Descriptions of experiences with the software in question would also be appreciated.

I have open office, audacity, Inkscape, GIMP, and blender...only thing is I don't really know how to use blender :P

Another worthy thing to mention is any game made by Valve (a game developer company) that runs on source engine is completely open source. The engine it self is also open source. If you want to find out more google "source engine".

Those image editing things look neat. I might download them and just them for drawing. Well, I'm not much of a programmer, but I do mess around with Stencyl when I'm bored.

Open Office has been replaced by Libre Office since Oracle/Sun was being a little uptight about the "open source" nature, though they've recently moved it to the "Apache Foundation" licensing, which should maintain the OSS flag, but I think it's too little too late.

Blender is pretty fun. It's one of the reasons I'm learning Python, so I can make my own plugins.

I'm also a HUGE fan of Google Chrome for browsing. I use Pidgin, an open-source IM client. And Thunderbird for Email. Also a big fan of X-Chat for IRC, PuTTY for SSH connections when I'm stuck on a Windoze box (like right now).

Heck, most of the tools I use at work every day are OSS. Other than my editor (ActiveState's Komodo IDE), VMWare and Charles, I think everything else I use is open source, including the operating system (Ubuntu), and my tablet's/phone's operating system (Android).

One of my favorite sites is http://opensourcewindows.org/ which has a TON of open-source programs for Windows, but many of them are also available on other platforms (Mac site)

source engine is completely open source

After taking a look at the Valve site, I don't thing Source qualifies. It appears they make it available for free for people who own a Valve game, for modding purposes. Similar to what Bethesda does for its Elder Scrolls games. Valve appears to license their engine and will only discuss pricing under a NDA.

IcoFx is pretty good for customizing desktop icons and if you're looking for a good card game, 123 Free Solitaire is pretty good.

Not sure how much anyone here runs a webserver, but I use this one for running and testing PHP locally before uploading to a remote server.

xampp

After taking a look at the Valve site, I don't thing Source qualifies. It appears they make it available for free for people who own a Valve game, for modding purposes. Similar to what Bethesda does for its Elder Scrolls games. Valve appears to license their engine and will only discuss pricing under a NDA.

I never said it was free, i said it was worthwhile to mention. They may give it to people for modding purposes but it is completely open source. The reason all the programs you listed are open source is because they are community built programs and anyone can take the code and add on to the program for everyone to use. Firefox is open source for this reason. Fully developed products are made open source for modding, such as Source Engine.

Most Linux distributions are open source and free for download.

My favorite is Ubuntu, but Ubuntu 11 sucked so I had to go back to XP. D:

Fully developed products are made open source for modding, such as Source Engine.

My point was actually that I can not find any documentation on the engine actually being open source. It is typically a requirement for being open source that the code be accessible and browsable. There is a good guideline as to what make software open source here. Listed in their definition:

6. No Discrimination Against Fields of Endeavor

The license must not restrict anyone from making use of the program in a specific field of endeavor. For example, it may not restrict the program from being used in a business, or from being used for genetic research.

and

10. License Must Be Technology-Neutral

No provision of the license may be predicated on any individual technology or style of interface.

You can check out Valve's Source SDK EULA, but it dosen't really strike me as "open":

Whereas, Licensee wishes to receive, and Valve wishes to disclose to Licensee, an object code version of the Engine, the SDK, and other information as deemed appropriate by Valve, all on the terms set forth herein;

     Now, therefore, in consideration of the mutual promises made herein, the parties agree as follows:

1.     License.

     1.1     License Grant. Valve hereby grants Licensee a nonexclusive, royalty-free, terminable, worldwide, nontransferable license to:

(a)     use, reproduce and modify the SDK in source code form, solely to develop a Mod;and

(b)     reproduce, distribute and license the Mod in object code form, solely to licensed end users of Half-Life, without charge.

Italics added for emphasis

On the contrary, Valve actually appears to be quite protective of its source code, even going so far as to conduct FBI raids when it found parts of its physics engine had been released.