What is the best SaaS collaborative software / service ?

Basecamp is the best I’ve used. Overall I like it because the basics are simple to use and it still offers quite a few features. I can sign clients up for an account and they find it easy enough to use that there isn’t a lot of explanation needed.

They have a mobile web version straight from the makers or quite a few native apps developed by third parties.

There’s also an API if you want to create something custom and several companies that have developed products to add on to the core offering.

Define “collaborate”.

Sharing spreadsheets? Word processing? Calendars? Contacts? Chat? Video conferencing? Screen sharing? Remote desktop? Project management?

Well, basically my most important needs for a Saas are:
– Task management
– Calendar
– Project management
– Milestones
– Limited acces for clients
with support for several users and groups.

Native Mac, iPad and iPhone apps.