What is it? As the name implies it's a site, a very small one-page site for now, that is only about mashups: recent mashups, popular mashups, mashup tag clouds, and some other miscellaneous stats about mashups. It draws from the database here and summarizes (so you can get the same mashups in either place). ProgrammableWeb is about more than mashups so it seemed sensible to put that mashup-only information in a mashup-centric place.
Don't want to be bothered checking back there to see what's new? Then subscribe to the so-called Mashup Feed and get automatic notification of mashups as they're added. Which, as a few hundred readers and visitors have noticed over the past few weeks, is available here as well: via the Feed icon to the left of this blog and also at the top of the /mashups page. This can be handy because only a small percentage of the mashups and examples indexed here actually get mentioned in the blog.
One other related note: you can now also subscribe to APIs via the API Feed. Anytime a new API is added or updated here the feed will syndicate it. Again, more APIs are added to the database than I get a chance to post about on the blog.