I use Claws-Mail (née Sylpheed-Claws) for my email.
One nice feature I found a couple of months ago was subject-line rewriting: A number of mailing lists I subscribe to add a totally obnoxious prefix on each outgoing message. I've yet to think of a good reason to do this - I can only assume it's for far less capable mail clients. Or something.
This results in much cleaner message list. You can probably do this in procmail, but having it in the presentation layer has the advantage that the rewrite is non-destructive - pressing Reply constructs a subject line from the original.
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