ProgrammableWeb keeps you up to date with APIs, mashups and the Web as platform.
“ProgrammableWeb gets you the latest on what’s new and interesting with mashups, Web 2.0 APIs, and the Web as Platform. It’s a directory, a news source, a reference guide, a community.“
If you want to make a mashup, you need some data first. This post (30 Resources to Find the Data You Need) gives some idea where to start gathering your data to work with.