This probably requires an explanation.

Unix-based systems don't depend on the extension for telling what type a file is; they look inside the file instead (for certain signatures called ‘magic numbers’). In fact, there are no file extensions in Unix. Dots followed by three letters are just part of the filename.

That doesn't mean there are no conventions! Debian packages (which are also N900 packages) are ALWAYS named .deb.

In other words, you can install debian packages. That's it. (sort of)

Debian has a package called alien, which converts packages between various packaging formats. It's often used to install non-Debian packages on a Debian system. I don't know if there's a version of this for the N900, and I wouldn't use it anyway. Too many things could go wrong.

You can also unpack tar files (or uncompress/unpack .tar.gz, .tar.bz2, etc) and install files yourself. I wouldn't recommend this either if you're not experienced with both the N900 and Debian.