Someone on a different forum was asking about downloading tons of information since they will be out in the middle of nowhere for like, months at a time and they want to give game development and/or programming language stuff a try. I typed up a big reply to them, and I know we've probably been over nearly all of this in this thread already, but I figured I would post most of it here just in case anyone new finds it useful.
I would say GameMaker Lite (or the full version if you can afford it) if you've never used it, but (like any tool) it's probably most effective if you can occasionally use Google to look up forum posts with people having the same problems you are.
There are one or two NeoGAF threads that should have information and/or links. I think one guy was specifically collecting links on all sorts of game development topics, but I don't know if I can find that.
GameMaker means you can prototype something FAST. It's easy to use, and the IDE lets you both edit the scripting language code and manage all your image/sound/etc. resources. You might be able to download/wget/whatever the scripting API reference from the website/wiki/whatever, and that along with some example games should be enough to let you experiment. You may be able to find sample game projects you can download and work with, as well.
Other similar tools to GameMaker include Construct2 (HTML5 games?) and Stencyl (Windows, Flash games). I haven't tried Construct2 and didn't care for Stencyl much.
For sound editing, Audacity. For music, there should be some open source mod tracker stuff if you want to take the time to download and learn it. For simple 8-bit-esque sound effects, if you won't have internet then google for SFXR (downloadable tool) and BFXR (website but also downloadable) to generate sound effects. There may also be some tools to make sound effects that aren't so silly, lol.
For 3D games, I'd say go with Unity (there should be a free version) and/or UDK. Heck, download them both with examples. UDK has a ton of versions but as long as the latest version runs, you should be fine. If you have a crappy computer, UDK may be out, but Unity might still work for you. Unity MIGHT be able to export to a web player and/or Flash, but I don't know if that's in the free version.
If you have a nice computer I believe there's a MyCryEngine SDK as well. I know very little about it though.
I know there is a lot of official online documentation for the UDK, and maybe for Unity as well, but I don't know if there is an easy way for you to download it.
If you go with 3D, download the latest version of Blender and a bunch of tutorials, guides, videos, and/or whatever to learn how to use it to make 3D models.
For images, make sure you have GIMP (despite its interface, search for Gimpshop if you want it to have a better interface) and Paint.net. Maybe get Inkscape if you want to experiment with making vector art without using GIMP's crazy path stuff.
If you want to stick with 2D games and you're stubborn and don't want to use a tool like GameMaker, I would recommend Java, C++, or C#.
With Java, you can download API stuff for Java itself (in complete HTML form, this is something nice about Java), and also look up information (hopefully with examples) for Slick2D. That's the library I'd recommend with Java, though there are other ways to mamke games with Java. Java has NetBeans and Eclipse for IDE options (code editing, code completion, project management etc.). NetBeans has a really nice GUI designer if you are laying out list boxes and buttons in a Windows application, but it comes down to preference.
With C++, I've worked with SDL and SFML, though there are probably like 10 other random game libraries you can find with various levels of complexity. I'd say that SFML is a good mix of utility code that doesn't force you into a game structure, without requiring you to be TOO low-level. SDL seems like it might be a little more low-level to me. I think both are cross-platform for at minimum Windows/Linux/Mac. If you get SFML, get the 2.0 stable snapshot and figure out how to build it on your system. There's API documentation online and maybe a way to download it too.
You should probably also look up some OpenGL reference material, since with both SDL and SFML, you can mix in OpenGL calls if you need advanced stuff to happen.
Lastly, with C#, there's XNA (get the 4.whatever version if I recall correctly). You download it from Microsoft and look up some examples. I think there's a big RPG example, should be a sidescroller example, maybe others, but make sure you get the ones for the latest version.
If you don't know Java/C++/C# very well, try to find some general language references for those that you can download in HTML form or whatnot.
As far as I'm aware, the free Visual Studio Express C++ and C# versions will work great for basic game development stuff with the libraries I mentioned above, or XNA for C#. Visual Studio has nice debugging and breakpoint support as well.
VERY IMPORTANT: With any of the options I have mentioned above, make sure that you can run everything involved. For instance, if you want to go with C++ and SFML, make sure you've built the SFML 2.0 snapshot on your computer, you know how to set the library path and link to the libraries, and that it runs. With XNA, make sure you can build a sample program without any errors due to missing .NET libraries or whatever. With Blender, make sure you can run it and export a model into Unity/UDK/both, especially since you -might- want a specific export plugin for Blender to UDK. Or that might be fine now, who knows.
This might be helpful for getting SFML set up:
http://sfmlcoder.wordpress.com/2011/05/18/creating-a-first-sfml-project/
This example looks like it was for SFML 1.6, but I imagine it can be ported to SFML 2.0 pretty easily:
http://blog.petermares.com/2011/08/16/picking-a-library/
The point is, if you are going to try to do this with very little internet access, you need to make sure your stuff is working ahead of time or you will be both offline AND sad.
Best of luck and let me know if you have any questions!