Steam Wallet lets you buy money?
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.