Building mashups is one of the hottest topics of Web 2.0. The term mashup originated in the music world—a music mashup is a remix of two or more songs to create a new song. A web application mashup combines complementary functionality from multiple web sites, e.g., combining Google Maps with Craigslist apartment listings to display on a map all of the apartments for rent in a particular neighborhood. You can create powerful Web 2.0 mashup applications in hours or days, instead of months or years if you have to start from scratch.   
    Developers have created mashups using Web services and RSS/ATOM feeds from scores of popular Web sites including Amazon, Google, Microsoft, Yahoo!, FlickR, Del.icio.us, Salesforce.com and many more.
    Our Mashups Resource Center focuses on the enormous amount of free mashups content available online. Start your search here for tutorials, documentation, books, articles, blogs, directories, tools, forums, etc., that will help you quickly develop mashup applications.

• Check out the newest and most popular mashups, including scores of Google Maps-based mashups showing you the locations of theaters, real estate for sale or rent, properties that have sold in your area, and even the locations of the public restrooms in San Francisco! 
• Use a really cool Web 2.0 Mashup matrix tool with links to loads of mashups. For each of the sites listed, find the mashups that have been created with the other sites in the matrix. 
• Search for mashups by type including mapping, photo, video, search, newsmashing, shopping, sports, travel, messaging, news, transit, real estate and more.
• Download the Web Services Mashup presentation from the O’Reilly ETech Conference.
• Read the Wikipedia article on mashups.
• Check out the FlickR APIs for adding photos to your applications, updating photos, replacing photos, example requests, and asynchronous uploading.
• Check out the article: "Building Mash-ups for Non-Programmers."
• Check out the Smashforce tool that enables Salesforce.com users to mashup applications such as Google Maps with their Multiforce and Sforce enterprise applications.
• Learn key mashups technologies
• Find mashup sites such as Givezilla, Podbop, and Strmz.
• Check out IBM's Enterprise Mashup Tool. 
• Get a graphic of the mashup ecosystem.
• Check out the Virtual Earth mapping tool from MSN that combines local search and mapping.
• Use Technorati APIs to find all of the blogs that link to a specific Web site, search blogs for mentions of keywords, see which blogs are linked to a given blog and find blogs associated with a specific Web site.
• Use the Backpack API to help organize tasks and events, plan your schedule, collaborate with others, monitor your competitors online and more.
• Use TypeKey authentication services from Six Apart.
• Use  PayPal for payment services.
• Use the Collaborative calendar API. 
• Check out  a mashup using Microsoft’s Virtual Earth mapping combined with FlickR photo-sharing to allow users to store and browse photos by geographic location.
• Install this mashup tool on your system—when it senses that you're browsing a book online, it pops up on your screen and allows you to quickly search online retailers to find the best price on the book.
• Use a Yahoo! Maps and event listings mashup to show you the location of events by date in a geographical area.