One of my favorite mashups is the Google search mashup.
You can see it here at the top right hand corner of the site; where you enter a normal Google search work or phrase and Google serves the result, mashedup, back into your site.
The same is true on The CEP Blog, where there the same mashup is used in one of the WordPress widgets on the right side at The Complex Event Processing (CEP) Blog
So, if you have a web site I recommend you use this mashup.