- Keyword discovery
- SEM Rush
- Wordtracker
- Word mixer
- Yandex Wordstat (English version - wordstat.yandex.com)
- Related searches in Bing, Yahoo and Google are great for keyword insights
- Google Autocomplete feature also helps with finding keywords frequently searched for
These are great for insights/keyword brainstorming. For a more thorough keyword research analysis of competition and search volumes and SEO profitability ratio calculations, look into Rank Tracker free version (there is a paid one too but the free one just fine for keyword purposes, the tool is part of seo powersuite).
Some other things to consider:
- misspelled versions of your keyword (really depends on your niche but works very well in certain niches)
- analyze your traffic logs and Google Analytics traffic stats - very good keyword insights there specifically for your site.
You can then analyze those keywords with Rank Tracker, come up with more SEO-valuable keywords if need be and then connect your Google Analytics account with Rank Tracker to monitor how much traffic each keyword brings + check your organic and universal search rankings in 750 SE's.