I think the best tool is Google keyword tool and it's free, and recently they just change their interface. just type a keyword phase such as " how to cure hemorrhoids" and it will show you related keywords by relevance. Pick top 10-15 from the list, and those could be a great LSI keywords. Just my thought
I always believe that if you need to run a successful and profitable web site then you must research your keywords before you burn up your cash reserves. The free tool from Google only shows content from its own adwords bidders. However, there is not much options for you but to manually research all keywords using that tool. There are a few tools only with hefty monthly fees of $300 per month. Being a noob/beginner it may turn out to be expensive for you.
The Google Keyword Tool works wonders, also, if you want to get more keyword ideas, try using the Google Wonder Wheel. I absolutely love it. It works wonders for me.
If you've got a site that's already up and running and getting traffic, hittail.com is a very good tool for extracting your longtail hits.
Google keyword tool external is a good one. But I will not give you long tail keywords Usually it provides only one-two long keywords max. But tha's not enought. Google stats is based on advertisers (adwords), so it is better to use some keyword databases, like in this post of Marketroid http://forums.digitalpoint.com/showthread.php?t=1732751#post14405693