I have to admit I was tipped off on this by a friend this morning (thanks, Scott!) - I was well aware of the really good job Yahoo had done with their AJAX framework (I worked on an application recently where the Yahoo libraries were used extensively), but this was something very few in the open source domain have managed (take my word for it now - I will google it up later ;)).
So what is Pipes? Remember unix pipes where you "pipe in" the result from one command to another and so on to create an aggregate of all the commands? This is a little bit like that. All you need is access to the feeds of whatever information you want to mashup in one of the standard formats like RSS, KML and mash it up in the interface provided by Yahoo (see here for more information). Interesting, easy to play with and looks to be going in the right direction. This is definitely one tool I want to keep on my radar for future mashup implementations.