This might be something a lot of people already know about, but I think it's a brilliant tip for anyone who uses the services provided by Google - Gmail, Google Calendar, Contacts and so on.
The "Mail For Exchange" app available either from Nokia's Ovi store on your handset or from this link doesn't only work with Exchange servers. It enables you to get your Gmail, Calendar and Contacts pushed to your phone and automatically kept in sync. If you read an email on your handset, it shows as read on your computer. Add a calendar event from your PC and it will magically appear on your phone... you get the idea. It's so quick that you can actually watch things change on your phone as you do them on your computer.
Once you've done this, you'll have an extra mailbox on your phone called Mail for Exchange - this will contain the most recent few days' worth of messages from your Gmail. It doesn't give you access to your entire mail archive on your handset - this would take up too much space - but gives you an at-a-glance look at what's new. If you want to see it on your phone's homescreen, just add the "Mail for Exchange" mailbox as you would any other.
One caveat I came unstuck with was on the first sync - I forgot to tell it to keep my contacts on my phone, rather than replace them with my empty contact list from Google, so I ended up losing all my contacts. Luckily, I had them backed up, so it didn't matter - but it's something to watch out for. Make sure you tell it "Keep items on phone" when doing your initial sync, and back up everything first!
It works so well for me on giffgaff that I've been able to use it as a replacement for sending expensive international texts to someone I know in the US; we can just use push Gmail over free data instead!