How do I show the available versions of a package in Ubuntnu?

I don't know how to do it in apt, but both Synaptic and aptitude (the ncurses interface) have a menu that shows all available versions of a package. In Synaptic there's a "show all available versions" menu entry. In Aptitude, simply view the package.

There are several methods, using the command line.

1. If you don't know the exact name of the package, you can use dpkg -l which lists all the installed packages, and pipe it through grep to isolate just what you want. Example: dpkg -l|grep firefox. This is what I use most often. Pro: helps you find stuff easily. Con: may get your more than one package (if there are several related packages with common bits in their name). But I still prefer this approach.

2. If you know the name of the package, you can use dpkg -s firefox. Downside: you have to know the exact name of the package, which is not always straightforward. On the upside though, this command is more powerful; if the package is not installed, it will tell you so; if it is installed, it will give you lots of info, more than just version (dependencies, description, etc.)