Keyword Discovery is also an option in finding relevant keywords.

Try wordstream.com - 3 different keyword tools
Keyword Suggestions
Keyword Grouper
Keyword Niche Finder

[quote=webvivre]
https://adwords.google.com/select/KeywordToolExternal[/quote]

and

[quote=UpstartAgent]
http://freekeywords.wordtracker.com
[/quote]

this two are the best..

I've found the Google Keyword tool to be the best. Also good for offering suggestions based on your website's content.

Wordtracker and Google Keyword tool are my favorites.

[quote=AlfredRE]I never understood everyone's fascination with Keyword generators. It's much better if you determine your own keywords through analyzing analytics programs. [/quote]
If you only analyze the keywords that are bringing you traffic, you might miss a lot of opportunities (that you might not know about if you don't do a little bit of keyword research). That is not to say that looking into your Analytics doesn't have its own purpose, not at all. It can be very useful as well.
I would say combine both. My favorite keyword research tools are Google's, Keyword Discovery and Wordstream.

I think the best keyword tool for the price is Google Keyword Tool. I really think you can get a lot of research done through it.