SEMrush is a very useful tool for both researching competitors when starting a site or for growing an established site. I really like to find weaker niche sites that still seem to be ranking for lots of keywords; SEMrush helps me see what they are ranking for and what I can potentially target. You can also see what keywords you’re on the cusp of ranking for with your established site - another very useful feature.
In addition, you can dig into the paid side of search and find out what keywords your competitors are bidding on, and then leverage those keywords for your own organic benefit if you're not already doing so. Search Metrics does this as well, but I've found SEMRush to provide a greater range of keywords and they save more historical keyword data than Search Metrics.
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.
Update: I’ve now been using Long Tail Pro for 5+ years. There are now many different keyword research tools and lots of people prefer some of the newer tools. Admittedly – Long Tail Pro had some hiccups and went through some rough times as they transitioned ownership and introduced the newer cloud version. But – all of the bugs and kinks have been worked out and Long Tail Pro is now better than ever! I’m happy and proud to be able to continue to use and promote Long Tail Pro as my keyword research tool of choice.
While this one isn’t necessarily a keyword research tool, it will give you valuable insight into how the keywords you’ve been optimizing for so far are actually performing for you. You might realize that you could start ranking for more keywords with more difficult competition, for example, or that only keywords on certain subjects are working for you.
For a long time, I wanted to develop a tool for content marketers that would assist in the process of coming up with blog ideas and subjects. Upon researching developers for this concept I had, I stumbled across Long Tail Pro, and realized instead of developing a tool myself for fellow bloggers and content marketers, I would throw my support behind this one.
For example, assume your search ad generated 5,000 impressions in one day, of which 100 visitors have come to your site, and three have converted for a total profit (not revenue!) of $300. In this case, a single visitor for that keyword is worth $3 to your business. Those 5,000 impressions in 24 hours could generate a click-through rate of between 18-36% with a #1 ranking (see the Slingshot SEO study for more on potential click-through rates), which would mean 900-1800 visits per day, at $3 each, or between 1 and 2 million dollars per year. No wonder businesses love search marketing!
The Google Keyword Tool is SUPER helpful for building a foundation for your keyword research strategy. At the end of the day, these search numbers are coming straight from the horses mouth. You can filter down to a hyper-local level and see which keywords are getting the largest search volume. Plus, with it’s integration with PPC you can get a quick idea about commercial intent by looking at the bid and competition metrics. How much are people bidding on KWs, higher = more likely to generate a return. Usually its aligned with search intent. That said, the trending data is a little less reliable. I would still use Trends to analyze the popularity/ seasonality of KW search volume.
Use the Keyword Planner to flag any terms on your list that have way too little (or way too much) search volume, and don't help you maintain a healthy mix like we talked about above. But before you delete anything, check out their trend history and projections in Google Trends. You can see whether, say, some low-volume terms might actually be something you should invest in now -- and reap the benefits for later.
The limit on manual keywords could be higher. I personally wish we could input 10,000 keywords at a time, instead of 200. However, I understand the costs that Long Tail Pro has to maintain each time a new manual keyword is input. Not a deal breaker, I just wish the limit was higher. (To be fair, I don't think most keyword tools have a bulk manual option at all).