Today only, MBNA are advetising fee-free balance transfer to any of their credit cards for existing customers, at whatever promotional rate you currently have. Mine was showing as 0% until July 2008, which coupled with no fees at all means free interest on the money transferred into my current account :-)
You probably don't even want to consider this unless you currently have zero balance on your MBNA card, but if you have, and are offered a low rate, then it's a rare bargain for credit card stoozers. Excellent also for people who have offset mortgages linked to their current account balance too.
Credit to neville72 on MSE forums for spotting this one.