There's been a lot of talk (or should I say "a lot of blog") about mashups in the sense of Musical Mashups, Video Mashups, Visuals/Pictoral Mashups, but little to no blog about Web Mashups, or mashups that utilize two or more separate URLs or sources to create either a new object entirely, or a commentary on the old sources. Some of these "web application hybrids," I find, are incredibly useful. Take The Hype Machine, for example. It is a mashup of all the "best" music blogs right at your fingertips. It holds them in a database and updates its site as the respective blogs update their sites. You can look at the Most Popular Blogs, the Recently Updated Blogs, or you can just type in an artist/song you like and look at the blogs that mention that person/song, starting you on a potentially never-ending trajectory into a music abyss.
On a more general note, ProgrammableWeb is the site where I found The Hype Machine and it is itself a mashup database of all the mashups and blogs, funneling countless like-APIs into over a thousand mashups into this one site. It is the patriarch of a multitude of mashup progeny. Another of ProgrammableWeb's displayed mashups is called Vidmeter, or a compilation of the top 100 viral videos, taken from AtomFilms, YouTube, GoogleVideo, MySpace, etc. FINALLY! I way to save time when I'm, uh, wasting time.
ProgrammableWeb also has links to sites like OpenKapow or to it's own How-To section to teach you how to make mashups. Now, I know very very little about APIs, HTML code or anything of that nature, but I'm sure someone with a little know-how could easily make their own mashup using two or more APIs and come up with something very useful and convenient. And, for those of you Mac users, a lot of these mashups sound like ways to compress a lot of info into a little space, something like a Dashboard Widget.