Show me the Wonga.com
I'm not going to insult your intelligence by pointing out the long list of "cons" of short-term loans - they have massive interest rates normally, are horrendously expensive on the whole and if you're so in need of extra credit, what about getting a 0% on purchases credit card or quickly clicking through to Zopa?
Well if you've already exhausted those options, a new(ish) player is doing the rounds. Wonga.com has just taken on a rather large cash injection of its own from venture capitalists so that it can make its business model even more efficient. Presumably this is because they expect it to grow...
Anyway, billing itself as a responsible lender, wonga.com offers very short term loans up to £750. As previously mentioned, you're looking at interest rates (or APR) of 2,689%. That amounts to a few quid in real terms and if you think you can get credit at short notice for a short term and not pay for it, then think again. Wonga.com are completely up-front about all of this and explain what their fees are, why the APR is so high and stress many, many times that they want to be responsible lenders.
Here's an example: borrow £135 for 15 days and you'll end up paying £161.29. If you don't pay it back in time, the usual procedures apply: you'll incur fees and the debt collection boys (or girls) will be round. The site advises not taking on a loan unless you know you're going to be able to pay it back on time and in full. Wonga is for people who know what they're doing financially but might find themselves with a cash flow crisis. The first time you use the service, you can only borrow up to £200 but each time you use the service and pay back your loan, your "trust" rating goes up. The maximum you can borrow is £750 but at such high interest rates that's just as well.
Although wonga.com has a consumer credit licence from the OFT, it's not regulated by the financial services authority. If something goes badly wrong, it can have its operating licence revoked by the OFT but there isn't the protection for consumers that regulation by the FSA offers. Perhaps it'll come under the FSA's umbrella in time - til then, your best options if you have to borrow cheaply are aforementioned credit cards. Or try your local credit union.