Custom css email themes aka "stationary", which other people can see on emails from you. These are still a very prominent part of email but I don't see anyone expressing their individuality with it anymore. It feels like a huge blast from the past to get one with any kind of theme that doesn't look like something a bank would use.

There was also "smileys" but I never used those because smileys were considered tacky. Yet it's the thing that stuck harder than anything else.