Adventure Game Studio (AGS) is an interesting tool which helps you to create your own 2D point-and-click adventure games.
Sounds good, but keep in mind that AGS is based on a program which first appeared back in 1997, and so your creations will look very similar to Space Quest and other Sierra games which were around at the time. Which means basic graphics, often at a very low resolution (many of the demo games currently available are 320x200, although you can create games up to 1024x768): there's little in the way of eye candy here, it's really all about the story.
Still, if you've fond memories of this type of game - or just want a quick way to try out some adventure gaming ideas - then Adventure Game Studio's simplicity should appeal. There are no heavy-duty coding requirements, and instead you start by setting up your scenes, creating rooms, objects and characters, everything that will make up your storyline. Then add some scripts to cope with game events, and you're done.
Of course there's the scripting language to learn, but it's not that difficult. There are plenty of games you can download, to help you see how they work, and a forum where you ask for help from more experienced users.
And once you've completed your masterpiece, AGS can compiled it to a single executable file so others can play it with ease. And if it's really good, then you can even sell the finished game commercially, subject to complying with the AGS licence. (The finished products look rather basic, though - if you can persuade anyone to part with money for yours then we'll be very impressed.)
If you're looking to create a new Halo or Call of Duty-type franchise, then Adventure Game Studio will be a massive disappointment. If you like old-style 2D adventure games, though, and would like to try out a few ideas of your own, then the program is an easy way to make this happen, feature-packed and with lots of assistance available.