It might be old, but it's certainly very widely used - both inside of the company and out.

Also, GMail is becoming more than mail - it's becoming the new desktop, in a way - with integration into mail, chat, calendar, and tasks... What they're doing with widgets will significantly change things, also. For example, Twitter can be integrated in one of those chat-box-like views.

I used to work on Google Translator, and the GMail team was very close in proximity to us. It's more than 10 engineers but probably less than 30... can't give you an actual count. But I do remember a whole bunch of them crazies running around trying to get us to be in their videos that they record (I'm not sure how many they've released, but I'm sure you've seen the one with the traveling envelope).

Amazing what small teams can do. Digg has more engineers than Gmail...

Off-topic, but seriously, what the fuck is digg doing with all their people and money? They've got a huge team and every time I venture over there, everything looks pretty much the same. Do they realize that digg is a sinking ship and they're all building lifeboats?

Wasting other people's money.

I count 8.

http://mail.google.com/mail/help/join_the_team.html

I'm not quite sure what you mean by innovation:

http://www.procmail.org/procmail.HISTORY.html