Google Adwords is basically the most used and trusted KW tool: https://adwords.google.com/select/KeywordToolExternal
I have read many good reviews about wordtracker.com I don't know if it because those people where affilliates, but I'm posting it here in case you want some more options, unfortanely it's a paid services, but since It's paid I think it comes with some automated stuff to make things easier to his buyers. There's also wordstream.com wich have some free keyword tools, like his free keyword tool, free keyword niche finder and the free negative keyword tool. Like alek and aira I'll recommend google tool above all, but I'm here giving you options, the choice is yours. Take care.
I might recommend the following tools: 1. Keyword Winner 2. Market Samurai 3. Keyword Elite 4. Micro Niche Finder 5. Wordtracker
You can get lots of free keyword suggestion tool online. Just search it in google and you will find them.
All keyword tools basically same in nature and how it's functioning. Google Adword Tool is the best in term of providing keywords suggestion and the demand for each keyword. To know the competition, you should do it manually on Google page by typing the keyword in search box. Other paid tools or service can deliver both (demand and competition numbers) automatically. This is only thing make it different from free tools. Any one you choose is not a matter (big) but the way you do a research is more important. By next month i plan to release my own tool for the public (free). It will perform both functions as what paid tools do. Cheers.
I also have to go with two of my favorites that @bogartkick already mentioned but you certainly wouldn't be sorry with either Market Samurai or Micro Niche Finder.