I just discovered that Molly was used by Jews for Miriam. I've always liked Molly, it's a sweet name.
Thoughts on it?
Other nicknames for Miriam that I like:
Mim- I didn't hear about this nickname until well after my daughter was born. Had I come across it I think I would have used it.
Mimi - My mom uses Mimi instead of grandma. I suspect we'd call our daughter Mimi if she didn't.
Miri - This is how I fell in love with the name Miriam in the first place. I met a woman called Miriam with the nn Miri. I loved it, but I have a niece named Mira. It would have been too much, in fact Miriam and Mira are confusing enough.
My daughter doesn't have any nicknames outside us, and we call her Miss Mae. Her second middle name is Mae and she's a force to be reckoned with. While I was pregnant I referred to her as Merry Mae, and my mom even called her that. After she was born it did not fit her at ALL. Of all the Miriam nickames Mim would suit her the best, it's simple, no-nonsense, and has a playful feel to it.