Edit - 2008-05-27: Updated to work with the (horrid) Reddit redesign. I can't be bothered updating the screenshots too.
On the main Reddit list pages, replace the "n comments" links with "x unread comments (n total)".
On clicking through to a page where you've already read some of the comments, jump to the first unread comment.
Highlight unread comments with a bright but non-distracting left margin.
My ulterior motive was testing the Gears DB with Greasemonkey. More than once I've wished it had a binding to SQLite, and with Gears it does: it just got a thousand times more useful. It'd be nicer yet if I could save to an arbitrary cross-domain database, but this is still a tremendous step up.
This script uses a bit of a hack and writes itself directly into the window, rather than just manipulating the DOM from the usual plexiglass sandbox. Strictly speaking it's not necessary, and only possible at all because I have no use for the
GM_* API functions, but a userscript with Gears does require at least some meddling of this kind.
Gears prompts the user to allow it to run on a specific domain, but the dialog doesn't appear if it's initialized from within Greasemonkey; it has to be done from the unsafe window. Once it's set up -- once the local database has been created and what have you -- Greasemonkey is fine, but that first step is critical.