Here are some tips that can be very helpful to market research:
Clickbank - high gravity products are a good indication that the niche is hot.
Amazon Best Sellers - you can find all the popular products in any category.
Ebay Pulse - similar to amazon best sellers. very useful for drop shippers.
Flippa Sold Sites - good indication on what is hot at the moment, useful if you are looking to flip your site in the near future
Google Trends - great indicator for current trends.
There are many more, but this are the ones I primary use for my micro niche affiliate sites.