I have found myself doing cross platform coding at work recently.
That is code running between different Unix platforms such as SUN, AIX, DEC, True64.
The differences can be plenty and subtle, but they are there in abundance. This is complicated by the fact that it was a decision long ago to use the particular platforms native compiler, not just one like gcc.
Some of the other similar products, like the one I am on, also have a version for windows.
Finally, just a few days ago, I grabbed the opportunity to get the product to build and work on RedHat Linux, so work is sort of interesting for awhile.
As for games:
C3, the game project I am on, was considering using the Torque engine, which is cross platform ready for Windows, Linux, and Mac.