2) SEMrush- This tool offers fantastic competitive research around domains to find what keywords could be driving traffic for your competitors. Looking at paid keywords ad spend can also help you know which keywords might have monetary value worth pursuing organically. If a competitor is willing to spend a high ad budget on terms and you think they do a good job running their ad campaign, then its a good indication it is worth organic ranking effort.
I actually don't use any keyword tools aside from Google Trends, but only rarely do I even use that. I try to talk to many of our target audience members (entrepreneurs) as I can. I attend events, I have phone calls, I sit next to them while working. Generally speaking, I think it's a waste of time to START with keyword tools instead of actual customers. Yes, you can target people in broad swaths and get a high level sense for what's interesting and trending, but at least in the case of our business at NextView Ventures, it's way more powerful to talk to actual "customers" you serve.
In a nutshell, Long Tail Pro helps you quickly find keywords in bulk based on a seed keyword that you input.  In addition to returning hundreds of related keywords, the tool also shows search volume (monthly search volume), advertiser bid, number of words, rank value, and my favorite Keyword Competitiveness (helps you judge the competition and keyword difficulty).