The terrific Terraria added Steam Workshop support in a patch yesterday, making it easier to install mods and share worlds with other players. Players have been modding the crafty sandbox explore-o-stabber for ages, and the Workshop doesn't support the most complex mods, but a Steam Workshop is certainly welcome for reduced faff.
Terraria's Steam Workshop offers worlds, texture packs, music packs, and language packs. So you can give things new looks and sounds, play it with new words, or just go see what other people have made. I am seeing: lots of golf courses.
Texture packs might be more varied than you think, mind. Along with reskins of lots of monsters and weapons and new world textures and such, people do use this to tweak the UI with new fonts and styles and things.
Developers Re-Logic are also curating a selection of their favourites, which include some wee official texture packs from the game's artists plus more golf courses.
For Terraria mods which change more, you'll need to go beyond the Steam Workshop and use the quasi-official mod framework. We've got some hot picks for the best Terraria mods to provide motivation.
Re-Logic announced earlier this month that the game has now sold over 35 million copies. They also noted that they still hope to add cross-platform multiplayer, eventually, maybe.