We've been integrating Drupal into our web development offering lately. It's really impressive!
If you don't already know, Drupal is an open-source content management system (CMS). It's amazingly friendly to development, which makes it perfect for creating custom-designed web sites with a non-proprietary CMS. Developers all over the world support it and it's the CMS of choice for sites like the London government, Mensa, Kofi Annan's foundation, Intel, the Grammys and the White House, too!
As designers and developers we love it because it's flexible enough that we can make it do almost anything. Clients love it because they don't have to buy it, pay licensing fees or get locked into one vendor for life while getting a powerful and capable CMS. Everybody wins.
The more we use it the more we love it and we're really loving what it can do for our clients.