By default the N900 comes supplied with only 2 Nokia repositories enabled, and the Maemo Extras one disabled.
All you have to do is go into App Manager and select the top menu bar, select Application Catalogues, select the Maemo Extras and untick the box marked Disabled. Refresh the catalogues and you’ll see loads of applications now, all the ones available via maemo.org.
If you’re brave enough you can also enable the Maemo Extras-Devel repository, the is applications in development and are put her for you to test. Think of it as being an alpha tester 😀
To add the Maemo Extras-devel repository just goto App Manager, select the tope menu bar, select Application Catalogues and NEW.
Add the repository http://repository.maemo.org/extras-devel using the same info as below, hit save and then refresh the catalogue.