IconVault's Chmod Calculator makes Unix permissions visual: click the owner/group/other read-write-execute matrix (plus setuid, setgid and sticky bits) and instantly get the octal number, the symbolic string (like rwxr-xr--), and the ready chmod command. It also works in reverse: paste an octal or symbolic value to load it into the matrix.

Handy presets for 755, 644, 700, 600, 777, and 400 come with plain-English explanations of what each one means. Free and runs fully in your browser.