Using appropriate keywords are immensely crucial to the ranking of the site you are working on. Imagine a site which has the coolest Flash applications, great and unique content, the best products and services. However, without appropriate keywords, it would hardly ever have a good rank in the Google SERPs! And what about domains and keywords listed in them? This is why Keyword selector tools help us in understanding and analyzing the market while also capitalizing on its benefits. Mentioned below is a list of 10 best keyword selector tools for SEO in alphabetical order: Ask: Run a search for say, 'Search Engine Optimization' in Ask.com. On the left side of the SERPs, you will find a segment titled 'Narrow Your Search,' that has links to all many search terms related to 'Search Engine Optimization'. Use these as keywords. Ask gives you refined SERPs through searching further with this feature. However, many may find this time-consuming. Also, it does not feature volume of traffic.
There is many free as well as paid keyword tools available on the web, out of them Google Adwords is best choice for keyword research, not because of it is free but it is more trustworthy than other keyword tools.
You can use adword keyword recharge tool or seo tools keyword density checker for keyword researching!