I just do a basic tomato sauce, just fry onion and garlic in a bit of olive oil then chuck in a tin of tomatoes, put in some mixed herbs or oregano, bay leaves, white wine and chilli, i just use the jar chilli, while the sauce is simmering i steam the mussels open in a separate pan with just a bit of white wine. once the mussels open i take them out and put them in the sauce, i strain the liquid the mussels have been steamed open in and add it to the sauce, give it another minute or two for the mussel juice to go through the sauce and serve with some chopped parsley.
I had a little bit of sauce left over from the last batch so i froze it and tonight chucked it with some prawns, pippies and mussels and fresh fettuccine.
Wish i would be more adventurous and do a white wine sauce with garlic and lemon or something similar which would probably be easy but i just love the combination of mussels and tomato.