Gmail plugin Rapportive is already one of the most useful tools for anyone wanting to get more out of their email, showing you information from the social networking profiles of people who contact you in place of Gmail’s advertising column. However, today’s it’s added a feature that could make it essential for anyone who uses both Google’s email service and Twitter – it’s essentially turned Gmail into a Twitter client.

While Rapportive previously let you see the tweets of people who emailed you, it’s now a lot more useful. For a start, you can now reply to people via Twitter, and retweet them too, right from Gmail.

There’s even a button allowing you to follow people, whenever you receive an email from someone you aren’t already following. This is particularly useful with new follower notification emails. While Twitter sends you information from a user’s profile within the actual email, Rapportive will display their latest tweets and let you reply right away, saving you the effort of clicking away to a separate Twitter client to find out if it’s worth you following back.

As you can see in the screenshot below, new followers just got a lot easier to deal with – you can see their recent tweets and follow straight back, right from the notification email.

Screen shot 2011 06 10 at 15.30.40 520x242 Rapportive just turned Gmail into a really useful Twitter client

In short, this is a real timesaver for anyone who spends a lot of time in their Gmail and Twitter accounts. While it won’t replace your usual method of accessing Twitter, Rapportive has succeeded in bringing the most useful elements possible of the microblogging service right into your inbox.

If you’re already a Rapportive user, your account will be upgraded to include the new Twitter functionality soon, or you can force an upgrade simply by visiting this URL: http://rapportive.com/twitter. If you’re new to Rapportive, head over to that link too, to get set up with this excellent free service.

As we recently reported, Google has planted itself on Rapportive’s turf with the launch of its own Gmail People Widget. However, the small US-based startup has fought back by rapidly introducing new functionality. If you weren’t already a Rapportive user, today’s new Twitter features should give you even more of a reason to try it out.