I've been exploring some mash-ups - web applications that combine data from more than one source into a single integrated tool.
For instance, RunningMap was great if you want to go for a run in New York but not so useful for Melbourne or Leongatha.
I know a dancer who lives in Detroit so I fed "dance" and "Detroit" into eparty guide and came up with 120 venue options.
mappr is obviously very useful for combining with other data.
I really enjoyed flickr Colour fields which picked out photos of various colours and shades.