APE tags
There are two versions of APE Tags. Version 1 (APEv1) is used by some versions of Monkey's Audio, is stored at the end of a file and allows for tags with any name.
Version 2 (APEv2) is used by (and originally developed for) Musepack, it is now also used in Monkey's Audio. It can also be used with other formats when using programs like Foobar2000 or Tag (a program that can create and read tags). It is comparable in functionality to VorbisComment. Just like VorbisComment it defines some standard fields, but it also defines some formats for those fields (like what a date should look like).
APEv2 also differs from Vorbis Comment in the way lists of values are handled. Suppose a certain song has two artists. In Vorbis Comment this will result in two ARTIST entries, but in APEv2 this will result in one Artist field with the two artists separated by a null character (a byte with the value zero).