Script Summary: Adds a "last read" (when not on a comic page) or "next comic" link (when at the read front) to web comics sites online, that keeps track of where you were, as you catch up with new strips since your last visit.
For comics, where you visit once in a while and never seem to remember how far you had read the last time, this script will remember where you left off, and let you pick up from there when you get back to the site, by always keeping track of the highest comic number you have seen, and adding a "[Last read]" link in the top left corner when you return to the site later on.
While the bookmark was designed to work with Megatokyo, Sinfest and Qwantz, it should work just as well on any comic using increasing numbers in the URL for its archive pages if you add domains for other comics sites using Greasemonkey's Tools -> Manage User Scripts editor.
Feel free to comment on what comic URLs you add, and I'll try to update the @include:s, once in a while, so new users won't have to repeat your manual labour.
Also note that if your last read strip was number 19, and you then view yesterday's strip 2109, then the last read bookmark for that comic will be 2109. To wipe 2109 and pick up at 19 again, go there and issue the manual override command Tools -> User Script Commands -> Set last read bookmark to this page.
Technical details: the [Last read] indicator will appear on all pages throughout the site, except on the latest strip you have read or later, where a "[Next comic]" link is placed instead, to go to the last bookmark of the next comic the script is tracking
Note that this does not include the index page (typically at the root of the site), which does not have a number, so you won't risk breaking your bookmark, when just entering the site.