Originally Posted by Jorophose
.dat = Appears to only be used in Simcity4. Must be for saving logs/records or something.
Actually, .dat is a pretty generic file extension used by a whole lot of programs. It generally denotes a some form of data file, but is not an actual format of any type. I've seen it used for data files in a number of games, old DOS applications, and even the Windows registry files, e.g. ntuser.dat.
As for .bif, that seems to be pretty generic as well
This is probably a good point to note that file extensions are not inherently meaningful. They're just conventions - anyone can choose any extension they want for any kind of file they want, and they often do. In the case of common extensions, like .dat, there is no anser to the question of what program can open it because, without more information, there's no way to know what the file contains.