One of the things I've noticed is that cubemaps (skyboxes) aren't as accessible as they could be. GIMP's cubemap utility doesn't actually work, and apparently Photoshop's utility isn't very intuitive either (I wouldn't know, personally, as I don't have Ps). It seems like a decent idea to get a thread going for cubemaps, similar to the one for models, so that people don't need to spend half an hour downloading utilities to get one made on their own.
I've got two done up already, and can do others on request (I end up doing them manually but it doesn't take that long in GIMP), which are posted below.
Any of the images in "Skies 360" on http://cgtextures.com/
will work, and I'm happy to convert them to usable cubemaps if people don't have the utilities themselves, just post the name. I have an account with CG Textures, so I can download whichever you need.