I have been beta testing this game environment called Atmosphir for a week now.
Atmosphir is a free game creation tool for creating 3d platforming games. The 30mb download comes with two modes - Design and Play.
In design you can use the easy and intuitive drag and drop interface to build a complete 3d environment with hazards and moving platforms... powerups , "auto save points" etc..
you start with this
go crazy with the platform props and hazards in the library
after youve built yr scene, you need to designate the start and end points of the game. the start flag is where your player will materialise when they load up the game, and the end/finish flag is their ultimate goal.
after your done, it takes a few seconds to upload your game to the servers,if you want to share it with others (you can just keep it with you and play by yourself...but wheres the fun in that?)
In Play mode you can choose to play from tons of games that people have already created as well as your own game..
I found the interface to be quite easy to figure out, alhough i wish they could figure out some way to allow creation of custom shapes and objects or even custom textures... but for now this itself is pretty good. and i assume the library of usable assetts will only continue to grow. If you can, do sign up ASAP for the beta testing programme, coz they only have provisions for a limited number of users.
heres another cool video from the site