Some sites you'll visit every day; others you'll visit when you get an RSS notification; but some you might want to hit only every Monday. Experimental Firefox extension My Weekly Browsing Schedule lets you automatically open up sites when specified days and times come around.
For each site you add, you can specify which day of the week and times it opens at. You can also specify sites to automatically load at startup, and tweak various other options. For really complicated setups, you can even import data from a text file.
The obvious use that occurs to me for this would be checking less-frequently used inboxes, or perhaps hitting your online bank to check credit card details. If you come up with a killer use case, let's hear it in the comments.