ssh-askpass | under X, asks user for a passphrase for ssh-add | Mehr ...
This is Jim Knoble's free implementation of the ssh-askpass program. . Jim calls this x11-ssh-askpass, but I've decided to call it ssh-askpass to make it easier to find, because this is almost certainly the version you want. Other ssh-askpass programs are available, some of which may integrate better into various desktop environments.