I use thunderbird at work on my ubuntu machine.
one thing that always annoyed me was the lack of a conversation view – one that let me see who had written to me and what I had replied. in-line. without them having to include everything in the reply.
basically the same as gmail. but inside thunderbird.
I found an excellent plugin for thunderbird that gives you just that:
thunderbird conversations gives you a clone of the gmail conversation view inside thunderbird!