I remember Konfabulator and Watson (is that what it was called? The one that was ripped off as a new version of Sherlock back when that was still a thing), but I think you're mistaken about why Appearance Manager never took off. If I remember correctly, Apple just never provided external developers with theme authoring software or documentation for the theme format, because between the original DP period and the public release, they(/Steve) decided that they didn't particularly want to promote wildly different UI styles. They never even officially released the themes they had demoed, other than Platinum.