Maths is a winner because it teaches you stuff that is very difficult to pick up on the job. It can start you off on a lot of paths. Even for seemingly irrelevant things, maths degree gives you an air of being a problem solving analytical type - which most decent jobs want.
In terms of examples:
one guy took maths to phd level. he's now working in the city, building computer models to trade complex financial products. Having such a concrete 'hard' specialism seems to mean the rewards are more meritocratic. Others seem to get much more bulls**t random stuff in interviews, which well coached kids from well connected backgrounds do much better in. He also works relatively short hours - his skills are rare and difficult to replace so they're nice to him. he loves it.
another friend works for the governement - something on the UKs defence/technology for the army
you can be a code breaker/codemaker. This is big business at the moment - internet security is just very sophisitcated secret codes. Likewise credit cards, mobile phone signals etc etc.
one friend works in 'risk' also in the city. You think about things that could go wrong, and what would happen if they did. you might have some rare things that would wipe out a bank - or many small things building up. You want to keep on top of your risks - a lot of people would suffer if a major bank went bust.
You can design computer games - find neat efficient solutions to problems to make the games feel natural and run smoothly.
you can study distant stars by applying mathematical techniques to try to make conclusions from the little bit of information that we can gather with our telescopes.
you can apply maths to scheduling things and optimising things - e.g. organising a big complex project.
you can apply maths to improve the design of things - anything from cars (my mum's job, incidentally) to medical drugs.
I've a maths grad friend who is now working as a research economist trying to find ways for developing world countries to improve their economies.
you can stay as an academin and learn about mind bending things like n-dimensional objects and countable infinity and imaginary numbers.
Utterly wonderful subject. If you're inclined to it, it won't let you down.