Sure thing. Some custom tags are mainly used to expose MB tags in the editor (Maybe there is just a better way to handle this)
Media Type CD, 7" Vinyl, etc.. straight from Musicbrainz
Record Date Date of recording (especially useful for lives)
Release Type ep, single, album, etc.. straight from Musicbrainz
Key Initial Key
Country Release country straight from Musicbrainz
Catalog # straight from Musicbrainz
Musical Objects Notable musical instrument or feature (yes the naming sux)
Recording Type demo, live, cover, etc...
Involved People Involved People List from Musicbrainz
Set Subtitle from Musicbrainz
Band Origin Country, region and city of Origin of the band or the artist
Status Release status from MB
Recording Location mostly useful for lives
Credit Artists Artists from MB, very useful for when sort artist doesn't cut it. EDIT: that's similar to what Hiccup does, i've likely stolen that idea from him at some point
Cover info Used for covers, contains Artist, album and release date of the original version
Composition date Used for classical music mostly
And yes, I know I have issues