Google Maps + a Wiki = WikiMapia (view of Bristol, UK)
Last.fm + Amazon ECommerce = TuneGlue Music map
Amazon ECommerce + your mobile phone = Ringfo
Hot or not? + captcha = Hotcaptcha
Digg + delicious + Flickr + reddit = Popurls
Google AdWords + Google Maps + hostip.info + Microsoft Virtual Earth + NASA + NOAA Weather Service + WeatherBug + Yahoo Geocoding + Yahoo Maps + Yahoo Traffic = WeatherBonk (with multiple predictions and live local web cams)
These and many, many more delights found via Programmable Web.