A structured wiki- and web page- creation tool offered by Google as part of the Google Apps Productivity suite.
Google Sites offers simple to build web sites, within a click of a button. There are numerous of templates available, both standard as in the gallery, and easily created by yourself.
Many items can be tweaked per menu (even with short cuts) but styling can also be done at HTML level.
Google Sites supports many possibilities to incorporate other Google Apps like documents, spreadsheets, presentations, picasa and more. Via an iframe gadget, it's possible to show content of other sites.
With the introduction of Google Apps Script (GAS) the possibilities seem endless in providing a professional intranet or website.