Personally I don't use Market Samurai as much these days and find the SEO module to be the most useful if anything- although it can be misleading given the many G updates which means that it is not necessary to build massive backlinking profiles these days... MS, Longtail Pro and similar tools all use G's keyword planner tool - which is good but at the end of the day is targeted to Adwords user... The issues that I have is that you don't get a great collection of semantically related words which potentially is money left on the table.... Hence I also focus on G's auto suggest within search and collate those as well as running words through a thesaurus... In addition time spent researching the niche such can yield other sub niches/categories... From there I then develop a content plan with use of silo....
I have been using SEMrush on a free trial for the last month and I LOVE IT! I will be paying the monthly fee for the pro version. I have never found a tool as awesome as this one. You can spy on competitors and see what keywords they are targeting and a ton of other analytics. It's especially helpful for PPC. If you want a free trial, PM me.
I've gotta warn you though, the paid version is $189 a month
I'm a MS fan, it work fine for me. But honestly, I don't think there's one tool that's way ahead of others, they're all good to help LTP, Google keyword tool and etc and etc. Just get one and get to work, that's the most important part.
Well, if you have a low budget, start with the Google keyword planner.
And if you wanna go pro, Market Samurai. Like Hassan said!
With a Wordpress site I place my keywords in the "Wordpress SEO by Yoast"