Gospelcom has had the Bible Gateway since Nick Hengeveld brought it with him from Calvin. He wrote it in college, and it was a glorious perl script at the time. Since then, it has become much more extensive, and even more powerful, largely thanks to Ron The Great.
One of the features that has been worked in is dictionaries that came with the various translations. Since they were so useful, the Gospelcom folk decided to make an actual external interface for the dictionaries, which you can find at http://dictionary.gospelcom.net/.
Since it just cried out for a bookmarklet, I made one. You can click the link below to try it, or right click it and save it as a bookmark. Or drag it to your bookmark toolbar. Or memorize the code and type it in by hand every time. You pick.