Hmm. That's a good question. Off the top of my head:
Mimi (
Marie,
Emilia,
Artemisia)
Mae (
Mary,
Margaret)
Lulu (
Talulla,
Louisa)
Elsie (
Elsa /
Elspeth /
Elizabeth)
Juni (
Juniper,
Junia)
Maisie (
Margaret)
Dru (
Drusilla)
Vivie (
Vivien,
Genevieve, Viveca)
Mo (
Maureen)
Hattie /
Hettie /
Ettie /
Etta (
Henrietta)
Effie (
Euphemia, Euphrasia /
Euphrosyne)
Jem /
Jamie (
James)
Mick (
Michael,
Cormac)
Mac (
Michael,
Cormac,
Malcolm)
Rex (
Reginald)
Bear (
Bernard,
Barnaby,
Baruch,
Oberon /
Auberon)
Pax (
Patrick)
Robin (
Robert)
Bertie (
Bertrand /
Bertram)
Cary (
Charles)
I'm sure there are scads more. But this would be my list for now.