I downloaded some e-books for my girlfriend's Kindle and was disappointed with the poor quality of meta data in the mobi files. I rapidly discovered that editing these files was not a trivial process. I downloaded Calibre, on the recommendation of everyone, but found it to be extremely wanting (* see below). I briefly considered MobiPerl, got as far as the description ("a collection of tools for generating and manipulating MobiPocket files written in Perl [emphasis mine]") and knew instantly that there be dragons.
(And, besides, I don't have any MobiPocket files written in Perl. Haha! Linguistic ambiguity is funny!)
Consequently, I spent the last two days building a simple, direct, no-nonsense mobi file meta-data editor. Download it, run it (no install necessary), open a mobi file, edit whatever data you see fit, and save it. It works just like you think it should.
Requires the Microsoft .NET Framework (v4.0).
It should probably go without saying that this has not been extensively tested. It hasn't corrupted the dozens of mobi files I've used it on but I can't guarantee that it won't render yours inoperable. I heartily recommend backing up your mobi files (oh, heck, just back up all your files) before you unleash the awesome power of the MOBIeditor on them.
v1.0 Original release. Add, edit, delete mobi meta data (specifically: EXTH header records).
v1.1 Added ability to quickly open the next/previous MOBI file in the current directory.
* (November 27, 2018) I have since, in the past six years, changed my opinion of Calibre. It is still overblown and way too feature rich, but I now find it to be a reasonable tool for managing my eBook collection. So that's what I recommend now, and will be putting no further development into this here MOBIeditor.