GPU miners
GPU's are used for mining other cryptocurrencies like Litecoin and Ethereum. These can be traded for Bitcoin or fiat currencies.
Some other cryptocurrencies have algorithms that are designed to be hard for GPUs and ASICs (cryptonote for instance) but I think it's just postponing the inevitable. If those altcoins become valuable enough I'm sure people will start making micro-optimized dedicated hardware to mine them and eventually it won't make sense to use GPUs for them either.