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.