Well a Google buyout of Twitter was shot down, but that doesn’t stop the internet gorilia from using the service. Here are a number of Google accounts set up using Twitter, visit their blog post for the complete list:
twitter.com/Google – our central account
twitter.com/GoogleCalendar – user tips & updates
twitter.com/GoogleNews – latest headlines via Google News
twitter.com/GoogleReader – from our feed reader team
twitter.com/iGoogle – news & notes from Google’s personalized homepage
Ads-related
twitter.com/AdSense – for online publishers
twitter.com/AdWordsHelper – looking out for AdWords questions and tech issues
twitter.com/AdWordsProSarah – Google Guide for AdWords Help Forum
twitter.com/GoogleAnalytics – insights for website effectiveness
twitter.com/GoogleAdBuilder – re building display ads
twitter.com/GoogleRetail – for retail advertisers
Developer & technical
twitter.com/GoogleResearch – from our research scientists
twitter.com/GoogleWMC – Google Webmaster Central
twitter.com/GoogleCode – latest updates for Google developer products
twitter.com/GoogleData – Data APIs provide a standard protocol for reading and writing web data
twitter.com/app_engine – web apps run on Google infrastructure
twitter.com/DataLiberation – our initiative for complete import/export of all data
twitter.com/GoogleMapsAPI – about using Google Maps embedded in websites
twitter.com/GoogleIO – Google’s largest annual developer event
Culture, People
twitter.com/googletalks – notes from our @Google speaker series
twitter.com/googlejobs – the voice of Google recruiters
Founder of Profectio of business sites which includes 
Great list. There were a few I didn’t know about, and they’re always great for catching the latest news. There’s also http://twitter.com/googlestudents specifically for students
Angie Haggstrom
Senior Copywriter & Founder, Angie’s Copywriting
Thanks for adding to it Angie.