For tarsnap (online encrypted backups), I'm using C99 (no, not C++, not C#, not D -- just plain C) and am aiming for POSIX compatibility on the client except in a few places where OS features make this impossible (e.g., reading file flags). On the server-side I'm using C99 and POSIX + BSD extensions (the d_type field in struct dirent is the first example which comes to mind).
Depending on your application, you may find that other languages are more suitable -- on the server-side, for example, you might want a language which does more to protect you from foot-shooting -- but I don't think you'll be able to find anything more portable than C.
Portability (python isn't nearly as portable as C), performance (there are some parts of my code which are very performance-critical), security (the most likely avenue of attack is via side channels; C allows me to make sure that the compiler isn't being clever in stupid ways), and familiarity (I've been using C for the past decade).
Depending on your application, you may find that other languages are more suitable -- on the server-side, for example, you might want a language which does more to protect you from foot-shooting -- but I don't think you'll be able to find anything more portable than C.