What are the options for sweetening?

This question relates to a tomato based sauce, but I imagine it applies to all dishes.

I recently made meatballs in a tomato sauce; typically, I find that any tomato sauce is too bitter. I found a recipe that suggested boiling a halved onion and butter in the sauce for almost an hour, and that worked really well.

I've also tried adding sugar, which obviously works. My question is: what are the other options that can be employed to make a dish sweeter?