NYTimes 538 as it aggregates polls and adds in economic indicators to decide who will be more likely to win various elections. Electoral Vote is a new one that Darin Ott told me about. It takes various recent polls and aggregates them. The main page has the electoral college (299 to 203 with 33 ties) as opposed to 538 which has 303 to 235 (both favoring Obama). Both sites also have the US Senate races (here for Electoral Vote and here for 538).