I just started using this product and I still haven't been able to shut up about it. I'm driving everyone nuts about it and just can't stop thinking about the potential. I have been on the internet since its inception and this product is revolutionary. I tell my friends about this and they say.. have you tried such and such. The answer is YES.. I try everything and this is one of the few times I have seen a product and a group with their sh(#$ together. You guys are on top of your game. Great work and keep it up.
I have been a fan of open source and this is the first real collaboration tool that has been released that is a real contender to things like 'sharepoint'. Finally... I can go into an organization with a tool and a pricing structure that is realistic.
So, with all that said. There are other tools out there.. good open source mailers, web imap interfaces, calendar programs... but what stacks up to Zimbra. Be brutal.