It's easy enough to collect a large number of site bookmarks to read on a rainy day... and promptly forget about them. If your favourites bar is starting to get cluttered and you're not in the mood to check each on individual, clean-up tools exist for both Firefox and Chrome to save you the effort.
For Chrome users, look no further than Bookmark Checker. Along with a clean, friendly user interface, Bookmark Checker comes with all the features you could want:
Scans for 404 bookmarks, time-out links or 403 favorites. Scan for duplicate folders. Scan for empty folders. Scan in several threads. Set timeout setting for bookmarks
404 Bookmarks for Firefox, on the other, gets straight to the point. No fancy graphics, just a basic page listing all your broken bookmarks. It's a little clunkier to use and can find false-positive (that is, still-working links), but it'll do the job.
Bookmark Checker [Chrome Web Store]