Where can I find a list of all formulas available for homebrew?

Online

You can visit formulae.brew.sh.

If you just want the package names for all formulae:

brew search

The following command will list the info snippets for all existing Homebrew formulae:

brew info --all

Or browse the local Git repository—thanks to Mk12 for that:

find /usr/local/Homebrew/ -type d -name "Formula" -exec ls -1 {} \;

Apart from the things slhck mentioned, there's an online package browser available at formulae.brew.sh.