Is there a convention that says where I should put programs I download if I want to make them available to all users? Blender, for example, appears to have a compiled executable, but also other directories and files that it (presumably) depends on.

The answer to To which directory should I install a program from a bash script? recommended /opt, but it isn't in my $PATH by default, and my /opt happens to be empty despite installing a lot of stuff from Ubuntu Software Center.