Lingro looks like the perfect online dictionary. Russell Stannard has informed about it in his May newsletter.
The idea behind Lingro is that we write in the box the address of the website we want to read (step 1). After that, if we come across some vocabulary item we don't know, simply by clicking on them we get an online definition (step 2).
This tool can prove extremely useful when reading online articles with a heavy vocabulary load, such as some of the articles we sometimes publish on this blog.
Give Lingro a try to gauge for yourself how useful this tool is.