Can anyone recommend a good web-based mass email tool?

I am responsible for sending out mass email communications for several groups and organizations. I really, really dislike using my Apple Address Book to store hundreds of addresses of strangers that clutter my personal contacts. I am looking for a web-based replacement that would allow me to create lists and send an email to everyone in that list. I would also need to be able to important the contacts that I already have in Address Book (so it would need to support imports of vCards, CSV, or something like that). Any ideas?