Google X was an extremely short-lived Google Labs project. It appeared in Google Labs and was taken down almost immediately after, with no comments from Google.
Google X made the Google search engine resemble the Mac OS X dock interface. When you moused over the different Google tools, the picture grew. The bottom text even said, "Roses are red. Violets are blue. OS X rocks. Homage to you." Judging from the quick and silent removal of the service, Apple may not have been flattered by this imitation.
Even though it's no longer hosted by Google, you can still find many Google X mirrors.