Whilst waiting for my mobile number to be ported to my new iPhone I needed a temporary phone on Pay as You Go to ensure my number was retained. The cheapest option with Orange was a Nokia 1661 for the princely sum of £10 plus £10 top up credit.
For such a cheap price the Nokia 1661 is an absolute bargain. It is very slim and light and has incredible battery life. Obviously for such a low price there are some compromises - this is primarily the keypad which isn't very sensitive but for a basic phone - for phoning and texting people it is probably the best value out there. The Nokia 1661 also does not feature bluetooth so it could be somewhat of a problem if you regularly travel in a car and need a handsfree car kit.
It uses a standard Nokia charger so can be easily recharged should the need arise, although with a battery life estimated to be 20 days this won't be needed very often!! In 7 days of use the charge has dropped to around 60% which seems exceptional - the iPhone 3GS needs charging every day!