Open Public:
http://openpublicapp.com/
(Open source Drupal like whitehouse.gov)
Achieve City
http://www.achievecity.com/
(SaaS version of Open Public tailored for muni)
City of DeLeon
http://www.cityofdeleon.org/
(WordPress theme developed by Luke Fretwell of GovFresh)
+100 to all the folks who said to be sure you have a long term service plan