Half-Life 2: 20th Anniversary Update

For anyone who wants a creative hobby, grab the game, download the SDK and start making maps/mods. It took a lot of effort to craft beautiful map, but comparing to later engines, HL2 is still easier to learn and make maps for.

(DOOM and Quake are also good targets and both have a large community)

HL1 is also good for a hobby, if not better. There are open-source alternatives to the engine and better editors, and the off-steam community still delivers!