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.