if you want to find popular product keywords try google products search home page, yahoo shopping, shopzilla, ebay and amazon.
Popular keyword would be very difficult to get high rank. What I suggest is a long keyword but you should get many backlinks to support your keywords rank.
I put together a listing of methods and tools for finding trends and key words. They are all FREE. Finding the best keywords and trends
I recently started working as a SEO i read it some where that the keywords should be related to kind of product or anything that you are promoting.
For finding best and high rated keywords try Google adword. Just write your key phrases or keywords Google adword gives you suggestion about best single and combination of words for these key words.
It depends what you're mean by popular. Something that is popular is not necessarily a marketable niche to target. It would be horribly saturated with other sites already well ranked and competing for it. What you want to do is go to Amazon and look at what's popular to get an idea of what products sell. Cheers!
I'm looking for something free. Where should I put the keywords in my website? Does it matter how many times it will appear in my website?