Canned tomatoes with too much vinegar
Just add a dash of liquid honey: it's a complex sugar that doesn't need much to combat an acidic taste.  If you have no liquid honey and the pasta sauce is still hot enough, use solid honey. if you have neither, use molasses, then cane sugar and if you only have white refined sugar, use that instead.
In all cases, stir well before tasting as you'll end up with ketchup if you add too much!
;-)