As a developer, I frequently have to clear my browsers cache, and also cookies, in order to test a site. This is a PITA as now I'm logged out from Google, PivotalTracker, etc, etc.
It also showcases how very few sites store login information in localStorage by default (note to devs, if you're to offer a "Remember Me" button, use localStorage, not a cookie).
So my idea is this: a set of two bookmarklets: the first would retrieve any form data entered in a form (prior to you submitting it) and store that data in localStorage, then the second would fill out a form using the data stored in localStorage for that site.
What about security you might ask? Well, clearly this should only be used on a personal computer - and maybe password fields should be excluded in any case. But this is stored locally, it is not transmitted anywhere, and the data is not accessible to any other site, so the data should stay between you and your computer. One exception would be any potentially malicious script hosted on the site, but that seems like a risk in itself - the same script could much more effectively simply grab the form data on entry.
So - good idea or bad?