Gmail interface tweaks tend to be all or nothing: you either keep the standard Gmail interface with all its bells, whistles and distractions, or you hide all the links and sidebar doodads at once. The Minimalist Gmail Chrome extension is different: it gives you control over 40+ individual elements of the Gmail interface, from the top bar all the way down to the footer.
Minimalist Gmail is mainly useful for hiding parts of the interface that don't do anything for you: the invites box (why is this still around, anyway?), the legal section in the footer, the links at the top to other Google sites, and virtually anything else you can think of. It's got other perks, too: you can replace the links in the Google Bar with custom links of your own. Mousing over each option will show you the part of the interface it affects, so you can see what you're about to hide.
Lifehacker calls Minimalist Gmail "the best Gmail tweaker for Chrome yet," and they're probably right. With this many individual options for fine-tuning, individual obsessives will find certainly find something to nitpick here, but the average user who's annoyed with Gmail clutter will absolutely love Minimalist Gmail.