So first of all, this isn't original. Credit goes here. But it's fantastic, and I wish I'd known about this a long time ago. As usual, for my own memory/use: and actually, I'm just going to clean up what the other guy said. He did a great job.
On the Internet-less computer:
In the terminal enter:
PACKAGENAME=<The name of the Package to install>
and then
apt-get -qqs install $PACKAGENAME | grep Inst | awk '{print $2}' | xargs apt-cache show | grep 'Filename: ' | awk '{print $2}' | while read filepath; do echo "wget \"http://archive.ubuntu.com/ubuntu/${filepath}\""; done >downloader.shA ready-to-use downloader for the package has now been created in the home folder. Open your home directory in the file browser and move the file downloader.sh to the top-level directory of your flash drive. Then eject your flash drive.
On the computer with Internet:
Insert your flash drive, and open your flash drive in the file browser. Copy the location of your flash drive:
[CTRL]-L
[CTRL] C
Move into the directory of the flash drive. In a terminal this time, type:
cd [CTRL]+[SHIFT]+V
Run the downloader:
bash ./downloader.sh
Wait for the download to complete and eject your flash drive.
Back to the Internet-less computer:
Open your flash drive in the file browser. In the browser, type the following to copy the file location of the flash drive.
[CTRL]-L
[CTRL]-C
Move into the directory of the flash drive. In a terminal this time, type:
cd [CTRL]+[SHIFT]+V
sudo dpkg --install *.deb
That's it!