Various free SoundFonts, including piano and General MIDI (GM):

• musical-artifacts.com

• polyphone.io

• archive.org

• List of SoundFonts with downloads available from a Google Drive folder

• kvraudio.com - this forum article is itself an extensive compilation of links for SoundFont files

• kunstderfuge.com - classical music, massive selection

• petersmidi.com - classical & pop from the past; not free, except for some samples

• bushgrafts.com - jazz

The links below offer a mix of piano and non-piano MIDI, which is a bit challenging:

• partnersinrhyme.com 

• thejazzpage.de 

• bitmidi.com