One of the best freeware games that I know of. SubTerra is a game where you control a miner in a brown (or sometimes purple) robe who runs around a top-down dungeon collecting gems and dodging hazards. The goal of this game is to collect enough gems to reach the quota, then run for the exit to complete the level without hitting anything dangerous, because if you do...
...you EXPLODE!!!
When exploring the levels in this game, you will be sure to run into vast underground oceans, fields of fire, hordes of Cryo Birds and Skelwings, lasers, bombs, balloons, portals, and the frightening Skull of Death. Some of the levels will have you running knee deep in glue away from a tide of green flowstone, taking your reflexes to the test, while others will twist your brain into several different kinds of knots!
The game comes with hundreds of pre-made levels to explore, and includes a full level editor so you can build your own dungeons.
While there is no online mode, levels can come with a second player, whom you can cooperate with.
There is also a sequel of sorts, called SubTerra 2. It is identical to the first one with the exception of a different array of items, blocks and features. It would be best to have both of them, since one has puzzles that the other does not.
Give it a try, by going to the download link below.
http://crystalshard.net/subdown.htm