The Uzbekistan currency, the Som, is quite interesting in a number of ways.
Firstly, because of the instability in the Som and its high rate of inflation, the US dollar is used in parallel (i.e. you can pay for most things in both currencies). There is an official exchange rate and also a black-market exchange rate (or the more appropriately named unofficial exchange rate, as everyone exchanges openly at the unofficial rate). The official exchange rate is around 1100 Som to a dollar, whilst the black-market exchange rate is 2800 Som to a dollar, which means you can exchange over 150% more on the black-market.
Now, you can further save because you have the option to pay in dollars or Som, for example an official hotel or attraction will have a dollar entry cost, but because they are official, they will use the official exchange rate and so paying in Som will save 60%. On the other side, an unofficial hotel will use the local black market rate, which as a tourist you will have difficulty obtaining, so it is marginally better to pay with dollars.
Secondly, the highest denomination is a one thousand Som note. If you exchange a single 100 US dollar note at the black-market rate, you will get 280, one thousand Som notes! This is a stack about an inch (three centimetres) thick. To pay for lunch, hand over around 25 notes, taxis between major cities, you better start counting the notes about five minutes before you arrive at your destination. I have never seen restaurants with the need for note-counting machines till Uzbekistan. We saw one man without a wallet, but a wad of folded notes straight in his back pocket. As Sonya mentioned, imagine having to pay for everything with fifty cent coins.