Video game music
Video game music rips: https://vgmrips.net/packs/
These are in VGM format, or sometimes VGZ when they're compressed with gzip. They may be converted to midi format with vgm2mid.
Websites
Online Sequencer is a useful website for creating, sharing, and remixing music in midi format. It is likely one of the most active midi sharing websites.
bitmidi is a midi sharing website mostly useful for finding popular contemporary music, video games tracks, and show tunes.
Tools
MIDI Transform is a paid program for Windows that may be used to re-scale a midi file's volume. It is available as a demo without file-saving ability and was created with .NET.