This one is a simple idea, and it may already exist. I’d like a way to maintain a personal blacklist of sites which I never wish to visit. When browsing the web, all links that (visibly or invisibly) point to these sites should then be somehow marked. Preferably, that should be done in a low-overhead way that doesn’t require rewriting every webpage. I’d be happy to hover over a link to see whether it is blacklisted or not, although color-coding or similar would of course be more user-friendly.
I do realize that some search engines offer blacklists as part of their “personalized search”, but I’d rather host this list on my machine and not have it tied to a global identity, requiring me to let Google et al. set eternal cookies to use them. Besides, this would also show bad links on other websites such as Wikipedia, which might come in handy.
One personal use for this is searching for lyrics. There are too many lyrics sites that are full of floating, animated, annoying ads of different types, some of them making their way through Firefox popup and ad blocking. Incidentally, this is the primary result of the music industry’s campaign against lyrics websites: the big lyrics archives which are left are scammers. Thanks, guys! (I was in the process of downloading all the lyrics from www.lyrics.ch when they shut it down. Unfortunately, I only ever got to the letter H ..)