I like the idea of Mashups - the problem is that most of the ones that I've seen tend to be demonstrations of how to use Wibble (s/Wibble/the_flavour_of _they_day_technology) - so the technology is usually quite interesting but the Mashup is never anything to write home about.
It was an even greater pleasure than usual to go the regular PHP London User Group meeting and see a Mashup application that I wanted to go home and use - straight away.
Andrew Betts' OnOneMap takes feeds from companies that sell or rent houses, he plots the property location on a map together with information about schools, stations etc. You can even find out if there is a mobile phone mast close by and until recently you could have seen the flood risk data, unfortunately, they have had to withdraw this very useful information. I am sure that this is completely unrelated to the fact that we have just had the wettest June on record in the UK.
If this site had existed last time we moved house it would have saved me countless hours of getting property locations from several different companies - all in different formats and then trying to plot them on a map. As it is I've just been using it to work out how much houses in our town are being sold for and whether I can afford to sell up and move somewhere less wet.
Oh, the technology? It's PHP of course