Early in the blockchain there are thousands of addresses with a single 50BTC block reward in them. Presumably people downloaded the client, mined a block and go

Early in the blockchain there are thousands of addresses with a single 50BTC block reward in them. Presumably people downloaded the client, mined a block and got bored. At that time you could only mine using Bitcoin-QT, which reuses the same addresses for each block reward. Most, if not all coins mined in this way are lost forever.