jemalloc is also pretty well-documented, though it's relatively big for a malloc implementation.

BSDcan paper: http://people.freebsd.org/~jasone/jemalloc/bsdcan2006/jemall...; Facebook Engineering post: https://www.facebook.com/notes/facebook-engineering/scalable....

The NetBSD version, though probably not the most up to date, is the easiest to understand, being in a single file: http://cvsweb.netbsd.org/bsdweb.cgi/src/lib/libc/stdlib/jema....