Is there a client that allows the "conversation" style display of emails like on Gmail?
Do you mean sorting e-mails by thread? If so, you're in luck! If you use Thunderbird, Outlook (Express), Mac Mail (and probably a bunch of others) you can do this with a few mouse clicks:
EDIT: If you're using Thunderbird, see comments in the above site about internally sorting the threads by date. Also, if you go to the linked add-on, and read the comments there, you can find out how to do it without having to install the add-on.
Separate names with a comma.