Estou aqui postando as contas do Google que já estão no Twitter, segue abaixo a lista delas, não é muito extensa.
twitter.com/Google
twitter.com/Blogger
twitter.com/GoogleCalendar
twitter.com/GoogleImages
twitter.com/GoogleNews
twitter.com/GoogleReader
twitter.com/iGoogle
twitter.com/GoogleStudents
twitter.com/YouTube
twitter.com/YouTubeES
twitter.com/GoogleAtWork

Localização Geográfica
twitter.com/SketchUp
twitter.com/3DWH
twitter.com/Modelyourtown
twitter.com/EarthOutreach
twitter.com/GoogleMaps
twitter.com/GoogleSkyMap

Monetização
twitter.com/AdSense
twitter.com/AdWordsHelper
twitter.com/AdWordsProSarah
twitter.com/GoogleAnalytics
twitter.com/GoogleAdBuilder
twitter.com/GoogleRetail
twitter.com/TechnologyUK
twitter.com/InsideAdWordsDE
twitter.com/GoogleAgencyDE
twitter.com/AdSensePT
twitter.com/AdWordsRussia
twitter.com/DentroDeAdWords
twitter.com/AdWordsAPI

Desenvolvimento técnico
twitter.com/GoogleResearch
twitter.com/GoogleWMC
twitter.com/GoogleCode
twitter.com/GoogleData
twitter.com/app_engine
twitter.com/DataLiberation
twitter.com/GoogleMapsAPI
twitter.com/GoogleIO

Cultura, Pessoas
twitter.com/googletalks
twitter.com/googlejobs

País ou Região
twitter.com/googlearabia
twitter.com/googledownunder
twitter.com/GoogleDE
twitter.com/GoogleKorea