Waz,

Looks to me like a globular South American cactus of which I know little about. It's definitely not columnar, AFAICS, so that rules out Trichocereus (even in its latest incarnation as Echnopsis). I think perhaps a Parodia, owing to the length of its long spines (and the arrangement of the radial ones) and the remains of spent flower pods up top. So, if you're looking for where to look, I'd guess Parodia first, Echinopsis second and something like a Gymnocalycium/Soehrensia third, but all of those could not be your cactus, which is a very nicely mounded round o' green.

Since I know of few SAC (South American Cactus) experts here, I'll link you to TOP (The Other Place), where I bet you'll get an ID of the right kind. You'll have to join up first to post a message.

Here is a link that might be useful: ID of the SAC at TOP