You don't own Terraria yet? Then stop reading, scroll up and buy it immediately! Maybe an extra copy for a friend as well if it's on sale.

Terraria is, hands down, one of my favourite games on Steam. I have put more hours into this 2.50€ Game than I put into Skyrim and DayZ!

Its one of the few games you can play with a screamish girlfriend while still being able to advance at good pace and having fun in the game. It is also a game that is fun when playing both singleplayer and multiplayer. There's always something to do!

Besides being long released as 1.0 , Red has kept adding new stuff to Terraria in slow but MASSIVE update packs (for FREE), which is really, really great and something you cannot take for granted nowadays.
If Terraria by itself starts getting boring - look into some mods like NTerraria which add Classes, Levels, Skills, scaled monsters, items etc to the game.