Game Maker
Game Maker is an innovative application for creating mainly text-based arcade games. It supports mostly GUI and command line applications and several game engines. Game Maker is multi-platform, it's free open-source, and with many source code under GPL. For those that are interested in developing games with text only, Game Maker might be a great idea to start out with.
With the new features of Game Maker, even a beginner can create exciting 2D games that can be played on Mac, Linux, Windows, and also on consoles such as Xbox 360, Play Station, and Nintendo Wii. It's also a good idea to know what type of technology your computer has. For example, Mac and Linux only support basic text based game development. If you have a modern desktop PC or laptop, you can probably get by with Game Maker. However, if you have a Mac, Game Maker might be quite a bit harder.
For those who want to try it out without having to buy it, there are versions for Windows and Linux and also for the iPhone and iPad. Game Maker was developed by Xeikon who is the creator of many popular software packages including the flash card maker called "Flash Game Maker". Xeikon is the creator of Game Maker: studio version which can be used in conjunction with the Flash software library of the same name which is available separately. For those that want to develop more advanced applications, there is the commercial version for the iPhone and iPad.