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?
Some quick things which you may or may not find in your nearby stores which can act as sweetners:


Honey
Jaggery
Corn Syrup
Some citrus fruits like oranges
Berries
Winter Squash like Acorn, Butternut