By accident, I closed my main Mail viewer window, and was looking for how to open it back up (not terribly obvious -- it's in the 'File' menu), and I found the Previous Recipients viewer (Window menu -> Previous Recipients) and tried it. It's a sortable and searchable list of addresses you have previously sent Mail to. You can also use it to clear out some or all of your non-Address-Book, auto-completing history of email addresses.
I guess this could sort-of also be used to check on activity for a Mail client that has deleted the items in its "Sent" mailbox. Chalk this up as another not-so-obvious, but pretty cool Mail.app feature.
[robg adds: Generally speaking, we don't run menu option hints, as I consider them to be 'obvious' and most users should discover them in their exploration of the app or system. However, sometimes I make exceptions for exceptionally useful items that may be overlooked. I asked a couple of Mail users about this one this morning, and neither had ever seen this window. So I chose to run it ... flame away! :) ]

