There are also some very good programs not found in the Ubuntu Software Center such as the Picasa photo manager from Google. You can download either the 32 bit or the 64 bit version of the deb files for Picasa at this link. The one for the i386 is the 32 bit version. The one for the AMD 64 is the 64 bit version. Use the one that is appropriate for the version of Ubuntu (32 bit or 64 bit) that you installed. Like other deb install files, just copy it to your Home Folder and double click on it as you would a Windows install program to install it. Of course, it will require your Ubuntu system password (your permission) for the installation to take place.
Some people like RealPlayer. If you are one of those individuals, you may be interesting in installing the RealPlayer deb file (at this link) for 32 bit Ubuntu installations ONLY.
There are other ways of installing programs (other than using deb install files) for the advanced user, and many things are possible with Linux. Of course, part of the fun is learning and doing the research yourself.
A note of caution is that not all deb install files are compatible with Ubuntu Linux. You need to research them to make sure any particular deb install file is compatible with Ubuntu first, and make sure that it is the appropriate deb file for either the 32 bit or 64 bit variety of Ubuntu. It may also, in some cases, work with only certain earlier versions of Ubuntu.