You have several options.
You can as with Windows search the net and find providers that have made .deb packages for their software, download it and install it at your leisure using dpkg or what ever GUI you have chosen to use.
You can also use
aptitude with the
download parameter instead of
install to just download the software you want and then take a backup of the packages you wish to use.
aptitude download terminator
The above command would download the terminator package so that you can install it at your own chosen time and place.
You can of course also download the source code, compile it and install it at your leisure. But the advantages of aptitude, apt-get, dpkg and other software that follows the Ubuntu distribution are many and makes compiling your own software for special circumstances mostly.
My suggestions all require you to use the terminal for downloading. When installing you can use any GUI that you have available for this purpose.