A Mash up is a Web site or Application that combines two or more seprate sites/ideas into one idea New and Original Site/Idea.
Last.Fm and Google Calender
Having data from Last.fm be imported directly into google calender can show what a person has heard and on what day and at what time they heard it. The Music Calendar can be seprate from the users ajenda Calendar(s), so it wont be too cluttered. The Music Calendar can then be shared with friends and family, taking music listening to a brand new level.
Google now tracks songs with google talk, this data could be entered into google calander under a new color.
Flikr and 1000 Words Wiki
A wiki that makes it easy to grab photos off Flikr then Allows for users to try and come up with 1000 words for the photo. It may be that good photos will have more then 1000 words that will be able to describe the photo. Each word that describes the photo can be linked to the definition of the word on the web someplace.
Wikipedia And Google Maps
A Mash-up that takes a wikipedia article, looks at what ip's have edited the article and then find there location and place them as a marker on a Google map. Wikipedia articles can show where people are around the world editing the articles. The logged in users on wikipedia may make for a not as accurate map. Popular articles with lot's of random, non-users editing it would be ideal. sort of exists: http://www.lkozma.net/wpv/index.html
Google Set's and Wikipedia
Using Google Sets and every liked word/phrase on a wikipedia page, suggestions could be made on other articles to check out based on what Google set's tells you.