Greasemonkey
Just installed Greasemonkey on Firefox, and it’s… wow!
The only problem — well, not problem; what makes it unique — is that it works only on client-side. So, you can write a script to correct a bug on your site, but I’ll only work for you. Except if you force your visitors to download it :-P
There is one comment yet. Don't be shy.
Well, if you know what’s the problem on your site, at least you can fix it ;). Worse thing, when it’s on another site…