If you're looking for a secret formula, you're not going to find it...
There is no one source that will tell you what is hot or not unless you go to that "hotornot.com" website.
In other words - "It depends" and "you have to do some research".
Are you looking for the latest and greatest product to sell as an affiliate? You could start by going to ebay.com or amazon.com.
Or are you looking for the latest hot topics for an adsense site? If so, you can start by looking at Google.com/trends and Yahoo's popular searches.
Hope this helps,