I discovered this series of videos though Sankaku Complex over the summer. They are basically compilation videos of every OP and ED of the latest anime season. The songs are altered so that they flow into each other, and when a song does change, the background image of the video changes to a picture denoting which show the song is from, along with the name of the song and the artist. The latest volume released so far is volume 7, which is separated into two parts, and it compiles every OP and ED songs of the summer 2009 season.
These videos are quite popular on Nico Nico Douga. For those of you who don’t know what Nico is, it’s basically a video sharing site in Japan similar to YouTube, but the catch is, you have to have an account to see any of the videos on it, which can prove to be a pain for us non-Japanese speaking individuals. Luckily if you type in “How to sign up on Nico Nico Douga” or something into Google, you can find a simple guides that direct you step by step though the registration process.
Here is a link to Anisong Mixxx Volume 6. which features songs from the Spring 2009 anime season. Sankaku is the only English site I know that allows for Nico streaming without registration. You can also browse through the Nico charts and search for videos on Nico there, and its all in English. So this is another alternative if you really want to watch Nico Videos aside from signing up on the actual site.