To be up front before writing about this company, I must say I rarely group e-mail. If I need to send something to several people, I can usually remember their names well enough to plug them into whatever e-mail client I'm using and get on with my life. There are, however, occasions when group e-mailing can save you some serious time, specifically the majority of business and enterprise users who rely on LDAP directories. That being said, if you are one of the people who takes a similar group approach outside of work, and would like to put together e-mail lists of your friends, there's a recently launched solution called Innercircle that helps you do it universally, without relying on a single client.
Similar to Zemble, which does the same thing for mobile users, and opt-in solutions like Yahoo and Google Groups, Innercircle lets you create a list of contacts that can be put inside a container e-mail address that goes out to the entire group. Each container gets its own special e-mail address that you can give a name and a description, and track using Innercircle's group management tools. In turn, your users can respond directly to you, or send a reply through the Innercircle address to send it back out to everyone. As group creator you can choose if your recipients can see one another's e-mail addresses, or you can send it to everyone like one big BCC.
The message management tool is really what separates Innercircle from a standard e-mail client. Assuming your contacts include the Innercircle address in their reply, you can keep track or reply history in each group page, although at this point it's incredibly limited, listing only the sender, date and time--but not the actual message. The service also lets you slurp in whatever contacts you've got from various Web mail providers using Plaxo.
In order to make money, Innercircle does one annoying thing, and that's stick paragraph-size ads with pictures on the bottom of your message. While they're not too obtrusive, going through a thread of messages with ads on them clutters things up.
While I appreciate the independence of using a specific client to set up groups, if you're the kind of person who only uses one e-mail client, you're better off sticking with what you've got. Gmail has a really simple-to-use contact manager that lets you put together groups, as does Outlook, Windows Live Hotmail, and others. Considering you've got to either save these special Innercircle addresses in your client of choice anyway, it's almost as much work as just creating the group in the first place. Also worth taking a look at is 9cays, which has been around since early last year and does nearly the exact same thing with a shared page where you can view the threaded conversation, complete with attachments.