Apple - How to know what packages are available with (home)brew?

brew search lists all available formula (aka packages), requires you to install brew first of course.

Additionally, if you want to get further information about that package before installing it brew info <package name> will list relevant information like version number, dependencies etc.

If you want an browser friendly option, there is the online http://searchbrew.com/

If you don't want to install Homebrew first (in order to see all the publicly available packages with brew search as patrix answered), you could browse the formulas on GitHub to see which packages are available — and how they will be installed.

Note: GitHub lists only the first 1000 files from a directory; which is far less than the number of Homebrew Formulae (3111 and counting). You can search for a known / partially known filename with file finder (or press t when at directory listing).