If you cannot see all the categories in the App Manager's Download section, you may need to activate the maemo.org/Maemo Extras catalogue. You can find out how to unlock maemo.org / Maemo Extras by clicking here.
Maemo.org Catalogue vs Ovi Store
Some people may wonder why there are two different sources of software built into the N900. The answer to this is in the origins of the Maemo Linux platform. Because Maemo so similar to other versions of Linux, and because there are so many open source enthusiasts using it, there are a lot of open source apps and games available for the platform. These are what you'll find in the App Manager's maemo.org/Maemo Extras catalogue, and that's why they're all free of charge as well as open source.
Ovi Store by contrast is Nokia's own commercial application shop, and it's intended mainly for commercial closed source software. That's why the selection of apps and games in Ovi Store is totally different to what you find in maemo.org.
Unofficial Software Catalogues
Maemo is an open platform, anyone can release software for it without having to get anyone's permission. If you only download software from sources like Ovi Store or maemo.org/Maemo Extras, it should be safe and reasonably reliable.
However, you can also download software from other less official sources which can be added to the Application Manager's catalogue system. These less official sources can contain a lot of interesting and experimental apps, but they are also generally much less tested so they may be risky to use. Just like on a PC, it's your own responsibility if an app from an unofficial catalogue does something nasty on your N900, so be careful.
