a more general question
Good idea, except a generalization would be for the operating system, not the device, since it's the OS that determines what software packages are available. By analogy, let's say I have an Acme X1000 Laptop, and I want to know "What software packages are available in what versions?" -- this is dependent on the OS I use on it. Generally, if an OS can be used on a given device, and a particular piece of software runs on that OS, then that software will run on the device via the OS, so the device is irrelevant to the question. Of course, there are inevitably exceptions and caveats.
Normative GNU/Linux distributions, such as Raspbian, Arch, and Pidora, are all really variants of the same operating system (GNU/Linux). So if software package foobar is available for GNU/Linux, it can probably be made to work -- by which I mean, configured and built without significant changes to the source -- on any normative GNU/Linux distribution.
However, "configuring and building" is rarely as fast (esp. on the pi) and easy as simply installing a pre-built package, and those are what the distributions provide. So we want to determine what packages are available for a given distribution in what versions. This can be done via the distro's package management system. Here's a brief overview of package management systems for the popular, normative pi distros:
Raspbian: Being Debian, the fundamental tools are
dpkg. For example,
apt-cache search foobar will help to find packages involving foobar; version information can be found via
apt-cache search --full or
apt-cache showpkg. Here's debian's own guide.
Pidora: Being Fedora, the main tool is
yum; you can search with
yum search foobar and all available versions will be shown.
Arch: The package manager is
There is a bit of a complication here in that package managers work by searching online repositories, and there may be more repositories available than your package manager is currently configured to check.
There are also GUI front-ends available for the above command-line tools, e.g, Synaptic for Debian and PackageKit for Fedora. Whether these really make things easier depends on the user.
I've left out RaspBMC, OpenELEC, and RISC OS, since I don't know much about them. I believe the first two are reasonably close to being regular linuxes and should have package managers of some sort. RISC OS is a world of its own.