C and C++, with a little bit of PHP.
C# is definitely only superficially similar to C and C++. It's too soft and fluffy. It is, however, now available on multiple platforms with the release of mono and the GNU C# compiler. They're available on FreeBSD at least.
C++ is, by the way, extremely cross-platform compatible if you avoid platform specific features.
wxWidgets makes even desktop apps cross platform.