There are few (if any) other places that offer as unique an environment as Critical Mass - I doubt I will ever work in another place where I have breakfast every morning provided by work, and get my hair cut at the in-house salon - on company time.
Critical Mass' clients have included Mercedes-Benz, Dell, NASA, Nike, Hyatt, AT&T Wireless, Procter & Gample, Albertsons, and Kaiser Permanante.
I was one of a small team of Web Developers working on a major redesign of the site in the spring of 2004, pushing heavily for use of standards (XHTML/CSS, separation of concerns, use of XML for content etc.) throughout the project. We had some late nights in developing the project, but the end result was good. We were later given a Web Standards Award from webstandardsawards.com in recognition for our work after I submitted our work to the "CSS Vault" and it garnered attention there.