Just so we have this somewhere other than individual threads:
Confirmed Twitters:
@alydenisof
@David_Henrie (Ted's son)
@lucyyhale (Robin's sister Katie)
@TheMandyMoore (Amy in Wait For It)
@britneyspears (Obvious I think!)
Unconfirmed
@jason_segel (Aly is following him, but sent him a tweet and is now starting to think that neither jasonsegel or jason_segel are really Jason and has called him out to prove her wrong! He's conversing with Russell Brand but there's not necessarily any proof Russell knows it's really Jason either!)
@joshradnor Only made a couple of Tweets, not confirmed one way or the other. Is following @jason_segel but not @alydenisof.
Definite fakes
@therealNPH
@NeilPHarris (He spells his own name as Neal and Niel in various different parts of the page. Come on now!)
Other
Accounts worth following for news etc that aren't a celeb:
@_himym_
@nphfan (I can pimp myself, right?
)
Anybody know of any other guest stars etc with Twitter accounts?