On Windows, I was using multiple identities in Outlook Express. While switching to Ubuntu I had planned on using Gnome's default messaging app Evolution. But while researching it I discovered it didn't support multiple identities or instances.
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On a Windows desktop multiple workspaces are a pipe dream. Your closest option is switching users. On a Linux desktop you have several options. You can switch users on Ubuntu too, or you could also run additional instances of X on other consoles Ubuntu automatically provides. Instead, I chose to take advantage of gnome's additional workspaces.