There are two interesting and recently evolving wiki documents on Wikipedia.org: Web 2.0 and Mashups, both are related to each other.

From the Web 2.0 wiki document:

Web 2.0 generally refers to a second generation of services available on the World Wide Web that lets people collaborate, and share information online.

From the Mashup wiki document:

A mashup is a website or web application that seamlessly combines content from more than one source into an integrated experience.

Content used in mashups is typically sourced from a third party via a public interface or API. Other methods of sourcing content for mashups include Web feeds (e.g. RSS or Atom) and JavaScript.

The etymology of this term almost certainly derives from its similar use in pop music where DJ's take the vocal track from one song and combine it with the instrumental track of another song resulting in an entirely new composition.

Many people are experimenting with mashups using eBay, Amazon, Google, Windows Live, and Yahoos APIs. The increased use and popularity of mashups has increased with the emergence of Web 2.0, which is characterized by active user participation and interaction.

Today I updated the Mashup wiki document to include the Windows Live reference in the paragraph above. The Windows Live platform group recently created a new Windows Live Developer Center at http://msdn.com/live/. The welcome message we included reads: The Windows Live Platform puts a deeper level of control into developers' hands by offering access to the core services and data through open, easily accessible APIs. Start building applications and mashups today with Windows Live.

I recently attended both the MashupCamp and Mix06 conferences where there were many demos and discussions around mashups.

There are many cool mashups on Via Virtual Earth in their site gallery. Some of the mashups in the Via Virtual Earth gallery are web sites and others are live.com gadgets. A good example of one of the mashups in their gallery is Virtual Places.
Virtual Places is a mashup that brings together Virtual Earth and MapPoint mapping services along with various other services. The idea behind Virtual Places is to highlight Virtual Earth in a mashup-style application. The application is implemented using ASP.NET Atlas, and features a complete mapping framework built using Virtual Earth and Atlas that features a programmable map control, overlays and pushpins. This is a truly excellent Virtual Earth mashup, and has set a benchmark for Virtual Earth apps in the future.
There are now over 200 gadgets free for download on the MicrosoftGadgets.com site in the gadget gallery. One which I saw demoed in a Mix06 conference session by Alan Lewis of eBay is their new eBay Search Gadget for Microsoft's live.com.
The eBay Search gadget provides a mini search window that gives you in-place eBay search results. Enter a query and hit enter or the Search button. Click Next and Previous to see more search results. Clicking on an item brings up the detail view, which provides the gallery photo, shipping information (if available), buy it now price (if applicable), and links to view the item on eBay or to add it to your eBay watch list.

We have open sourced the gadget, so you can download the source code and improve upon it or re-use the code in your own gadget. The gadget is built using eBay's REST API. More information on the gadget for developers is available here: https://searchgadget.codebase.ebay.com

A recently article CNet: Microsoft mixes software for business 'mashups' quoted Bill Gates talking mashups:

"Applications are changing in their architecture," Gates said, speaking at the Convergence 2006 trade show here. "We're taking the best elements of the online world, where we are seeing mashups," he said.