What applications commonly available for Linux also run on Mac and Windows?

One of the main strategies in breaking Microsoft lock-in should be to
replace the most popular applications with free ones.

If the user can run the same apps on different OSes, he can become less
aware of the OS. As the OS becomes less relevant to the user, there is
less objection in switching away from Windows. The rise of non-Windows
OSes increases the general demand for cross-platform apps. Web-based
apps also increase that demand.

We have seen a start to this scenario with the rise of several
large-niche cross-platform apps, most importantly Firefox, Thunderbird,
and OpenOffice.

The next step would be cross-platform implementations of smaller-niche
applications, such as those for image processing, internet conferencing,
and music composition, to name a few.

What is the current general situation regarding portability of free
applications, notably those now available for Linux and covered by
licenses similar to GPL?