Another one I didn't realise existed… IE7 has a simple way of adding other Search engines to the Search box.
Just choose the Find more providers… option.
The one I use the most is the Wikipedia option – it's very handy to search for a term on Google (my default engine) and the also check it out on Wikipedia just using the drop down.
(Firefox and other browsers have similar features as well.)
And as always, this Tip is covered under my 'well duh' Disclaimer.