The problem with that is dependency. One piece of software that you want could have 20, 30, 160, dependencies. If you download and do it by hand you'll be downloading all 160 (definitely an extreme case...) by hand.

You can download the deb's from maemo.org: http://maemo.org/packages/

Search for a package, drill into the one in the repository you want (extras, devel, or testing) and then download the "File: package.deb".

Once you try to install it; it will error on you saying it "requires" a whole list of other software, which you will then have to download one by one; and then each of those will require their dependencies.. etc.

Your BEST option is actually either setting up a Wifi Ad-hoc and using ICS (windows) or Firestarter (Linux) to setup internet sharing... or doing USB Networking.