Links to the official forum sections so you can modify your game as you wish:

Mods:
http://www.terrariaonline.com/forums/client-server-mods-tools.97/

Maps:
http://www.terrariaonline.com/forums/maps.101/

Texture Packs:
http://www.terrariaonline.com/forums/maps.101/

Feel free to discuss below if you want to point attention to certain mods you like ;)

PS: GOG current version (on release day) is the 1.2.4.1 version

There is also a way to make music packs. Better than just muting the music and playing your own because it stays context sensitive. I did one with Secret of Mana music which worked really well.

http://www.terrariaonline.com/threads/how-to-custom-soundtracks.21951/

Unfortunately I don't know how to do it for the current version.

Don't forget about the map editor!!

http://www.terrariaonline.com/threads/tedit-terraria-map-editor.35394/

If you can get that working with the current version, I daresay keep me posted.

I didn't forget it, it's always at the top of the mod section, so it's hard to miss it ;)

I use it sometimes when I want to try a specific artefact or cheat a little bit :o)

Trying to install tapi and tccl to the gog.com games folder and it says to make sure i have a legal copy of terraria....

Tried to install the Gameiki mod, and it wouldn't work--couldn't find a steam-related dll.

steam_api.dll

The mod developers for some reason decided to police pirates, and since the only DRM-free versions used to be cracked versions they think you pirated it. Best thing to do is to contact the mod developer and explain the situation.

Hello,

Here some of the options mentioned are broad and customize the overall visual appearance of Terraria, others are specific if only the selected elements are adjusted. Terrarias games are as simple as adding a texture pack. Most of these mods are available for free.