Zimbabwe dollar is no longer used because the Zimbabwean government mismanaged the currency by printing vasts amounts of it in order to pay off foreign debt and current expenditure. It caused hyper-inflation and caused the currency to devalue by 100,000% in a year.

They now use gold and US dollars to transact.

It's an interesting story and a perfect example of what can go wrong when a central authority has control of a currency.

http://chinwag.com/files/images/photos/2342854191_9f4794a1fd.jpg