Firefox/Chrome/Safari/Mailplane: Rapportive, a plugin which we've previously mentioned for its Gmail-enhancing powers, has recently added Twitter capabilities. So now you can follow, reply to, and retweet your contacts — all without leaving Gmail.
Once you install Rapportive, you'll see your contacts' information (including social networking info) in Gmail's right column. Rapportive's Twitter service is just rolling out, so you may need to visit http://rapportive.com/twitter to apply the upgrade.
After that, you can follow contacts on Twitter, reply to their tweets, or retweet their tweets from within Gmail.
As of right now, Twitter feeds aren't displayed for all of my contacts (whom I know have Twitter accounts). For those contacts where the service is working, though, it's a convenient way to keep up with them on one page.