Hi. There is a good plug-in for those who use Mozilla Firefox. It checks the words you type in the browser for mistakes and offers available amendments, so you don't have to bother yourself looking up in a dictionary in order to prevent mistakes. However, I wouldn't advise it for lame ESL learners like me, because you will get used to it and when you're at another desktop and has no dictionaries nearby, you have difficulties.
In brief, you can download it from here: https://addons.mozilla.org/en-US/firefox/browse/type:3