Dowloading something from maemo select gets me an .install file that links to a repository, I know this an internet device, but is this the only way to download programs? I don't have a wlan and using my limited data plan seems unnecessary

I wish there were some way to do this.

You can also download them manually (on a computer) and install the packages. If you open the .install file in a text editor, you'll see the repository url and the package name. You'll pretty much want to look here. Just find your .deb and get it. Maemo should know what to do with it
EDIT: you might want to get wireless or bluetooth internet on your pc. Manually installing has disadvantages, like not automatically finding and installing dependencies. It's much easier if you use the automated system via internet

AFAIK the application manager does not support installing local debs, so for that you would have to use command line root access via X terminal. Not recommended for beginners.

You definitely should consider getting either an unlimited data plan or a WLAN access point (those are quite cheap).

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The Application Manager can handle local debs.

No, the Application Manager on the N900 doesn't let you install local packages.

Time to visit an internet cafe that does have wifi.

If the local package handling has been disabled in application manager on the N900 then there's dpkg -i <package> from the command line, as root and all that. If you're uncomfortable with that then getting online is the only way.

If you haven't got Wi-Fi at home, and don't want to setup a proper access point (although that would be best), you can get cheapish USB dongles for your PC that can turn them into a Wi-Fi AP and share the PC's internet connection. It would mean your PC would have to be left switched on while using Wi-Fi on the N900.

Maybe it's time for a Windows application plus a Maemo 5 app that allow downloading the .install-file (and whatever it points to) on the desktop PC, transfer it via USB to the N900 and trigger the install procedure there.

Probably a Windows version would do. I simply assume most GNU/Linux users wouldn't need it.

Simply running dpkg -i <package> from the command line on an N900 that's offline can be tiresome if the package pulls in a lot of dependencies.

So, kinda like a package manager proxy that works on the PC? Could be an interesting project. To work well the PC app would need to know what packages are already installed on the device, so that it knows which dependencies need to be downloaded.