The first thing I did when I pulled my N900 out was set up and sync with Google over Mail for Exchange - works brilliantly, though you'll need to tinker with the settings or else you'll get constant 'server not responding' errors (seems Google doesn't like it when you ping constantly).

Contacts, Calendar, and Tasks seem to be supported, though it's only a single calendar, not multiple (would like to see multiple support really soon).

Note that Gmail is not supported as part of your Mail for Exchange profile (or didn't seem to be for me) so you'll need to setup the Gmail profile separately using the built-in Gmail profile.