What are some good .mbox viewers / editors for OS X? I have some .mbox files containing thousands of emails that were exported from Gmail. I'd like to be able to view mails, by conversation and by label (Gmail label), search, delete some mails, etc. 

Essentially, it's a mail app in terms of UI, with labels on the left and with threaded conversations, but it works with a static mailbox file instead of talking to a real server. Sort of like how you can open a text file in a text editor, read it, search, delete things from it, and save it back.

What are some good apps that let you do this?
Apple Mail allows importing from .mbox files (see the import option in the File menu). I don't know how well it supports the other requirements you've listed but it might be worth a try.