I assume N900 user? Additionally I assume Maemo5? This won't work for anything downloaded from ovi store fyi.

Save the .deb packages locally and copy them somewhere.
Then you can simply copy them back to the N900 and dpkg -i *.deb

As far as the packages you have installed go (provided you have not done a apt-get clean)

Something like this to backup to another computer
# scp /var/cache/apt/archives/*.deb user@server:/home/user/maemo/

Later
# scp user@server:/home/user/maemo/* /home/user/MyDocs/
# dpkg -i /home/user/MyDocs/*.deb

If you want to just download packages to install later then you can:
# apt-get -d install unzip
# dpkg -i /var/cache/apt/archives/unzip_5.52-14-maemo6_armel.deb