ProgrammableWeb has released a list of 'popular' mashups, which John Musser calculated from a mix of click-throughs and user 'votes' based on a 1 to 5 scale rating system. Here are the top 10 mashups according to ProgrammableWeb:

1. Virtual Places
2. Weather Bonk
3. Diggdot.us
4. Flash Earth
5. Adactio Elsewhere
6. Where's Tim Hibbard?
7. Elicit
8. 2RealEstate Auctions
9. Flickrmap
10. Streampad

All the usual API suspects are included in these mashups: Google Maps, Flickr, del.icio.us, Amazon. I tried out Virtual Places and was impressed that it includes little old New Zealand. Here is my hometown of Wellington:

I've been checking out mashups from other sites too, particularly Ning.com. A couple of note are Restaurant Reviews With Yahoo! Maps and Anytown Marketplace With Maps, the latest version of Jon Aquino's craigslists-like mashup.

Also the media companies are coming on board, led by BBC Backstage but also washingtonpost.com recently released an excellent site called mashingtonpost.com. Some interesting mash-ups that people have done already on mashingtonpost.com: News Cloud (a tag cloud), Ripped from the Headlines! (a daily news quiz), world map interface, thumbnail quiz of Arts & Entertainment stories, and washingtonpost.com search results via RSS.

This is an exciting time to be experimenting with mashups, whether you're a publisher, a 'user' or an API-wielding company. As Yahoo's Matt McAlister put it, "the idea is to make your content mash up ready and to build incentives for people to use your content." In that spirit, I've (finally) freed my own RSS feed under the Creative Commons, plus I'm currently trying my hand at mashups. Anyone can do it, so why not give it a try?