These will mostly be for linux users, but there are versions of most (except Terminator) for win & mac.
Zimbra - Email. I use it for email, but it has a lot more.
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Twhirl - Twitter client. Adobe AIR app so it's available to everyone.
Deluge - Bittorrent
Terminator - Terminal program. Allows as many vertical & horizontal splits as you want, which is handy if a program has terminal tied up, you can open another within the same window.
