There are actually quite a few screen capture apps that use this same idea, but Skitch is available for free, which is a considerable advantage.
E nwere n'ezie ihe ole na ole na-ekpochapụ ngwa ọdịnala na-eji otu echiche ahụ, ma Skitch dị maka n'efu, nke a na-ejighị ya eme ihe.