As anybody who owned a PC up until the mid-nineties can tell you, MIDI files were the default music tracks you could hear when playing a game. This technology was (thankfully) relegated when audio cards became available, but some of us developed a certain aural allergy to MIDI files that was never cured.
However, if you have no problem at all when it comes to music in said format and are keen on classical compositions to boot, you will probably enjoy a visit to the Kunst Der Fuge website. Broadly speaking, the site is an online resource that presents the user with a collection of free classical pieces as MIDI files. Some of the featured composers include Liszt, Mozart, Schubert and Beethoven. Moreover, it is possible to visit pages that collect files arranged by theme, such as “Renaissance Music” or “Sacred Hymns”. Registration has to be complied with in order to have full access to the featured collection, and a fee must be paid.