AGS is open source software and free for non-commercial use.
If you wish to make money from your game (either shareware or commercial) you should read the Legal Information to be clear on the license agreements of AGS components.
AGS itself has no license fee and is free for commercial use, bearing in mind the terms of the legal information page.
Yes, the AGS community is run by a team of dedicated volunteers, who put their time and efforts into keeping it running as a welcoming, friendly and informative place to be. The AGS server and forums are paid for out of our own pockets, so in effect it costs us money to provide a free service to AGS users.
If you appreciate the work we do, and would like to give a little something back, you can donate here.
There are official Linux/iOS/OS X/Android ports of the game engine, which can run AGS games created with v2.5 and later. You can find it on the Download page. There is currently no non-Windows version of the game editor - one may be created if there is enough demand for it.