Dr. Web Anti-virus Link Checker
Full user review
It's really fast, not intrusive at all...
If you have some problem installing it on opera, know that you don't have anything to download - you have to modify standard_menu.ini, everything is explained right there : freedrweb.com/browser/opera/?lng=en (copy n paste in you're browser) - it's quite easy and only takes a few moments...
Well, it's still Dr Web, can't really say they're the best around, but it stills a great tool.