So you have downloaded and installed a version of cube2? There are several forks out there including but not limited to Sauerbraten, Assault Cube, Blood Frontier, Red Eclipse, MekArcade, Sandbox (Moviecube) and the new sdl2 port of Tesseract which Octaforge is built on. In case that selection was not enough for you to enjoy, there's also the Bananabread java(script) port for browser based gaming with WebGL and Firefox.
The deal is you can come to Quadropolis aka. Quad and download/discuss the latest community contributed content updates, model and texture packages, sky boxes, mods, patches and scripts.
All the content published at Quad is rated and updated on a regular basis. While cube2 is completely free in all aspects so you can use it for both commercial and non commercial projects the content may not have the same flexibility. Make sure you check the content license and build version before downloading to minimise wasted time/bandwidth or legal concerns for professional usage.
If you are a Red Eclipse player then your life is made easier with in-game updates available and maintained by the "RE" dev team for your convenience. For the other builds/ports/forks you can keep up to date by coming back here on a regular basis to get your fix. Yes, everyone is well aware of the options for in-game updates. It is a topic that has been widely discussed. There are various reasons for and against in-game updates. Depending on your game server you can run map updates with setmap/getmap. Not everyone has high speed connections to keep up with large package updates and some server hosts prefer to keep the focus on gameplay rather than map building and modding.
If you are serious about mapping, modding and mayhem then you can find plenty of other people to collaborate with here. Keep in mind that most of the globally distributed content will end up here at some point. Quad seeks to be agnostic about the actual game engine and game play. It's a community effort where everyone is welcome to get involved. The main focus is on quality. So if you want to find the bugs in your work this is the place to get that feedback.