A personal favorite election indicator is the Halloween mask indicator, which holds that whichever presidential candidate sells more Halloween masks is the one that will eventually win the election. The indicator has correctly predicted every presidential election since 1980.
The reason it's my favorite indicator is that it's somewhat counter-intuitive (wouldn't you think that you would buy the mask of the candidate that scares you more--the one you aren't voting for?). It's also a favorite because of what it says about mind share, marketing and elections. Whichever candidate holds the greater mindshare both at the ballot box and at the costume shop is the one that ends up winning.
Thanks to Google trends, measuring mind share is easier than ever. The more that people are searching for a candidate, the more that they are thinking about him. Google trends only goes back to 2004, but in 2004 and 2008, the candidate that was more frequently searched for on Google is the one who ended up winning.
Below are the google search trends for Bush/Kerry, Obama/Mccain and Obama/Romney. Mccain never eclipsed Obama in searches in 2008, but in 2012, Romney has taken a slight lead out of the Republican convention. As the election heats up in coming weeks, this could be an indicator to watch.
Note the scale is slightly different on each chart.