Google's Gmail Labs has just rolled out a useful, but mildly creepy feature that gives you suggestions on who you should e-mail based on previous conversations. So, if you've had threads going with a group of people, it will recommend some of those folks once you've added at least two addresses in the recipients field. Best part is, they don't even have to be in a group of contacts you've created in Gmail's contacts manager.
I got this to work to an almost uncanny level. It handles things like family members without a hitch, but where it gets useful is in pulling up people you've recently been corresponding with and giving you a one-click link that adds them to the message. Short of organizing these people to a group (in which you may need to add or delete someone on a per-message basis), or doing a reply-all on an old message, this is a really fast way to get another thread started with the same group.
Now all Gmail needs is a quick way to take these contacts and add them to a sending group without leaving the message, something that can only be done in Gmail's contact manager.