Developer: Javid Alimohideen
Price: free Download on the App Store
Mail2Group is an adjunct to the iPhone's native email app. You can organize your contacts into groups and manage your groups in a variety of ways. With Mail2Group you can send an email message to everyone in the group, send email messages to only some people in the group, and you can decide to use the same or other content in the subject line for the same group.
You can sort your groups alphabetically, copy them to form new groups and set "copy" and "blind copy" to the people in the contacts that comprise the group.
In Settings, you can set a confirmation before launching your email, which might keep you from accidentally emailing all your co-workers your opinions about the new boss. Settings is also where you can also choose to sort your groups by name or to leave them in the order you created them.
Mail2Group is one of those apps that should have been included with the iPhone and that's probably going to happen sooner or later. However, it's hard to imagine that Apple could come up with any features that aren't already in Mail2Group. Alimohideen seems to have thought of everything. I didn't encounter any bugs either.