Fittingly coming in just ahead of Google Maps, web mashups are easily one of the most innovative web trends we've seen this decade. Traditionally, a web mashup combines the data of two sources and turns it into something unique, such as Yelp using their data with Google Maps to create a better representation of nearby shops and restaurants. But a web mashup need not pull from two different sources so long as it redefines that data in a unique way. For example, a website that pulls information from Twitter and displays it as a 'chat channel' by making use of the hashtags could be considered a web mashup.
The great thing about web mashups is that they have the ability to create very unique and powerful applications. For example, Weatherbonk pulls in information about local weather and displays it on a Google Earth map, which allows you to easily see the weather.
And the open APIs that allow web mashups to exist also power some of the best iPhone applications. After all, any time you see an application that is combining data with Google Maps, you are seeing a web mashup in action.