Rapportive support has been around for a while now. However, many people I talk to still don't know about it, and you should, even if you don't use Mailplane or Gmail. Rapportive takes your email address and provides the recipient of the message with an overview of your online profile based on social media sites where you use that same email address, such as Flickr, Twitter, Facebook, LinkedIn, and much more. There are a couple important things to note here. First, Rapportive is separate from Mailplane. It's a plug-in that you have to enable. Second, Rapportive is getting this information from public sources. However, you may be surprised how much public information is easily compiled when connected with your email address.
I've shown several friends what Rapportive shows me for their contact information, and every one of them has said, "Wait, I'm not sure I want that connected with every email I send," "That information is outdated," or something similar. Even if you don't use Gmail regularly, it might be worth checking out what Rapportive shows you. Just install it and load Gmail in a browser (or enable the plug-in in Mailplane). If you find there is incorrect information there, you can edit it yourself (for Gmail addresses) or contact the developers for other addresses.
Rapportive also removes the ads that normally appear in Gmail's sidebar. Overall, I find it really handy, but I would caution against assuming that the information that you see is 100 percent accurate and current.
Not content to merely add those features, Mailplane also adds support for automatically opening "safe" files. This feature is turned off by default (unlike Safari), but if you want to make images, movies, PDFs, and archives open as soon as they finish downloading, now you can.
If you'd like to be able to automatically set the BCC based on which account you use and the to/from addresses, Mailplane can do that now, too. There's also an OmniFocus plug-in -- it's not a new feature, but I had forgotten about it until recently.
Version 2.2 is a pretty significant update despite its minor numerical increment. You can download a 30-day demo from Mailplane's website. If you decide to buy, it's US$24.95, €21.00, £18.00, or ¥2495.
Naturally, you can always use Gmail in a browser for free rather than spending money on an app like Mailplane. But I've used Mailplane since it was in beta, and I would never go back. It is very actively developed, with new features regularly being added. Having a real Mac app for a mail client is well worth the money for anyone who spends a lot of time in email. If you manage multiple Gmail or Google Apps email accounts, the easy switching alone makes it worth the asking price.