User Ranks to individual members

[Gizmix]

Hello.
I am wanting to implement User ranks into my forums.

An example: Im wanting to give a 'donator' rank to everyone who donates, But without creating a whole new 'donator' usergroup for it. So the rank would be given to individual people accross different usergroups.

I have seen this on other sites (such as heavygaming.org , sythe.org), And assume there must be an easy solution / mod to this

Thanks for any help and advice.
sam