Dictionary.com Toolbar 5

It's very basic and simple, so don't expect something that will drastically change your internet experience from Dictionary.com's toolbar. But it is very handy for quickly looking up words. You can program it to either open a new window whenever you look up a definition (which may be intercepted by pop-up blockers) or to use the current window to display the results. You can also adjust the toolbar to use icons instead of icons with text, making the total length of the toolbar very miniscule, which you may find a useful feature. It's so small yet it's always there, and I use it regularly with ease.