Keyword Discovery has a simple and nice interface which shows historical and seasonal data as well. These include major international, pay per click, meta and regional search engines.
I mostly prefer google keyword selector tool. because another tool give us different result so if you want to optimize your website for major search engine then use only google keyword selector tool. I am not force to all of you to use this tool. but you use many tools to keyword research then you get confused yourself.
I tried many of them in past few years but then now i am permanently on a free keyword search tool which you can find out here http://www.serpanalytics.com/tools/top_keywords
I think Keyword Spy and SEMRush are the the best among all the keyword research softwares. Too bad it is not free.
This Keyword research and tracking tool can gather new keywords from such sources as Wordtracker, Keyword Discovery, Google Adwords, Yahoo! Search Assist and some more, don't remember exactly. Free version has some limitations, but it enough for work.
I use quite a number. 1. google keyword tool 2. free version of wordtracker 3. seo book (gotta love the way they line up everything in order) 4. semrush (useful to get cpc's and competition) 5. spacky - dumb name, great keyword tool Im sure youll get a thing or two from the ones i listed. Give them a try and supercharge your KW research!
E-marketing Helper For every keyword you can see : the search volume/month, google,msn,yahoo and altavista search results, price per click + many more.