Didn't see this posted already, there's a feature for Google toolbar that lets you translate any English word you read online into Korean by just scrolling over it.
http://translate.google.com/translate_tools
Its the last option listed on the page.
I've found it really useful in learning some high frequency words that I come across in my regular web surfing. Saves a lot of time over looking something up in a dictionary.
Once you install google toolbar just go to "settings", "options", and check the translator box. Set it to Korean and you're all ready. If you're like me and don't like looking at toolbars much you can set the toolbar itself to be hidden in Firefox.