How do I download software, besides the Ubuntu iso itself when you can't connect from an Ubuntu machine (e.g. from my Windows laptop)? I've logged in to Ubuntu One, then into Canonical from Ubuntu One, but clicking on Canonical's 'software' button takes me to a webpage with nifty graphics and a link to use for downloading the Ubuntu iso itself, but when I am at the webpage I can't download any packages that I would normally be able to install in Ubuntu from the Ubuntu Software Center.
Visit Ubuntu Packages Search webpage, and under the heading that says Search type the name of the package you are searching for in the Keyword: text entry field, select your distribution from the dropdown menu to the right of where it says Distribution:, and click the Search button to show the search results.
The search results of Ubuntu Packages Search may contain more than one package. Select the package you want to download, select the operating system architecture (either amd64 or i386), select a download mirror which has a location close to you, and download the package.
In addition to searching for packages in all the currently supported releases, Ubuntu Packages Search also supports searching for packages from the second latest release even if that release is an EOL release. For example in September, 2018 the gcc package from the second latest release, Ubuntu 17.10, can still be found at
https://packages.ubuntu.com/artful/gcc as shown in the screenshot. This is useful when you need to download and install a missing package for 17.10 in order to upgrade it to 18.04, even if 17.10 is currently an End Of Life release.