As you may have noticed, the search feature in Messages.app is pretty lacking. Finding a particular exchange in a conversation is clumsy, scrolling back is slow, and there's no way to save a chat. MessagesHistoryBrowser is a small app which attempts to fix that. It lets you search and browse comfortably and quickly through your whole Messages.app history, save chats as simple text files, and locate files (pictures) that were sent in them.
At startup, MessagesHistoryBrowser will import your Messages.app history, which may take a while depending on how large it is (typically a minute or so). Subsequent launches will just read from that import, but you can refresh it whenever you want with cmd-R.
MessagesHistoryBrowser requires Mac OS X 10.11 El Capitan and above, and is distributed free of charge. The code is available on github.
NOTE :I've received a few reports of users for whom the app crashes right on launch, while importing the Messages.app DB. The issue is hard to pinpoint because obviously I can't ask the user to send me their Messages.app DB out of privacy concerns. In any case, if this is the case for you and you have a bit of time for some tests, please contact me.
NOTE 2:Due to having lost the private key used to sign the Sparkle-based updates, you'll have to re-download it to get the new version, and you may get a weird message from the previous (1.0.1) version. That's normal, no maliciousness involved.