xterm has a secure keyboard option from its menu (control+click) that is supposed to prevent other applications from being able to receive X11 events related to input.

the thing you are referring to on os x is similar, with a system-wide capability. when you enter passwords for keychain and similar things, these have secure input enabled by default. i think it's up to the application to enable it, but when it's enabled for a field, no other application can intercept those events.