A very short but useful hint which not everyone knows: you can access your contacts directly from the N900 desktop by just typing them on the keyboard. You don't even need to have the Contacts app open, it will open automatically at the correct section when you start typing. See the video embedded below for a demonstration.
Click here for a complete list of All About Maemo's Nokia N900 Tutorials
(You can also see this video on All About Maemo's YouTube Channel.)