Scenario (for Windows): You’ve got multiple GMail accounts, say, one for work and another for personal use; and you would like to sign onto both Google Talk accounts on the same computer at the same time. By default, GTalk doesn’t permit multiple instances, but a simple hack can fix it.
Right Click on the Google Talk icon (in the start menu, either in your quick launch or, Start > All Programs > Google Talk > Google Talk), then click Properties, then add /nomutex at the end of the line, for example, in my setup this looks like:
“C:\Program Files\Google\Google Talk\googletalk.exe” /startmenu /nomutex
Now, launch as many Gtalk instances as you want. Voila! Have fun 😉