Many Web sites invite mashups by providing application program interfaces (API) to their services.
The API for each service is different, and often poorly documented. They generally require only basic programming skills, but figuring them out can be time consuming.
Google has made it a lot easier to learn their APIs by creating the Ajax API Playground, a very cool site for people who want to create mashups with Google Maps, Search, Feeds, Calendar, Visualization, Language, Blogger, Libraries or Earth.
The API Playground has over 170 examples, but they are not static, they are dynamic. Here is one of the examples -- the code is shown in the top pane and the resultant map below.
If f you change the value of the variable address, and click Run, the map will shift to the new address. Try it yourself.
Do you find these dynamic examples useful? Do you know of other dynamic example sites?