Traditionally, tools to manage file ACL entries have been setfacl(1) and getfacl(1). Those are command-line tools and some people can feel uncomfortable using them.
Eiciel allows you to visually edit file ACL entries. You can add and remove users and groups who will be granted permissions through the graphical interface.
You can have an idea of how it looks in the screenshots section.
Eiciel is developed in C++ with gtkmm-2.x. It is initially targeted for GNU/Linux but it also works in FreeBSD 5.x. Support for Solaris would be nice, but let delay it till they have a POSIX ACL-like interface.