Valve are in the process of rolling out a Steam Wallet thing for Steam. You can use it to add money to your Steam account in weird £4/$5 increments. Why? Because Valve are mad scientists , and they felt like it.
NowGamer took the time to work out what all the increments are for the UK, US, and EU territories:
UK: £4, £8, £18, £35, £70
US: $5, $10, $25, $50, $100
EU: €5, €10, €25, €50, €100
At least they have the common decency of representing the money with real currency - SteamPointz would have been a bit much. The funds added to your wallet are non-refundable and non-transferable, although you can still buy games the old fashion way.
You can get down to it here , but I'm not seeing an easy way to access it via the Steam client just yet.