Google Apps for Your Domain lets you offer private-labeled email, instant messaging and calendar accounts to all of your users
It's all free* and everything is hosted by Google. No hardware or software required.
Did you notice the asterisk? As asterisks go it's pretty benign, if you don't currently control a domain they'll register and host your domain for a reasonable $10/yr.
So, for your "organization" (I am using it as a portal for my family) you get a google-esque start page, mail services hosted by google and gmail interface, private calendar, gtalk capability, and a web-based wysiwyg "page creator".
Setup is painless for the internet savvy. I regged my domain through goDaddy.com for $6.95 with a discount so you could save some cash there.
It's a nice portal that will allow my family to communicate easier.
