For example "free stuff" is a search term that gets thousands of queries every day, but most of the sites in the top results have thousands of incoming links so it would be very hard to compete them, while a search term like "free cellphones" or "free computer stuff" could be easier to chase.
With that in mind I decided to write down a list of keyword suggestion tools. These tools can help you find information on many keywords (eg. how many times a phrase is searched per day) or find you some relative keywords (eg. "free stuff" is relative to "free ipod")
Bellow is a list with all the keyword suggestion tools I found. If I missed something please let me know
1) Google Adwords Keyword Tool
This is the official tool from google adwords. A great thing that this tool provides is the "site related keywords" which can help you see what keywords are probably used by your competitors.
2) Overture Keyword Selector Tool
This tool provides related keywords using data from Yahoo's ppc program.
3) NicheBot classic
This used to be my favorite tool. It is the original nichebot.com tool which displays information from overture and wordtracker but merged.
4) Trellian Free Search Term Suggestion Tool
Another good tool to get keyword suggestions.
5) Geoland Search Engine Queries
A keyword suggestion tool that displays data from several search engines.
6) 7Search Keyword Suggestion Tool
Another great tool from one of the bigger CPC companies, 7search. You can use it to get keywords and estimated prices (for the 7search network)
7) Wordtracker Free Keyword Suggestion Tool
Generate up to 100 related keywords and an estimate of their daily search volume.
8 ) DigitalPoint Keyword Suggestion Tool
Using data from overture and wordtracker
9) Niche Watch
A tool that displays the competition in your niche. Requires a free account to use it.
10) CheckRankings Suggestion Tool
This tool is similar to google suggest tool (I think they use their data)
11) MS AdLabs Search Funnels
An interesting tool to see how people search a certain keyword and analyze the search behavior that users have.
12) BRL Keyword Suggestion
This is one of the best tools I found. It uses data from 7 sites to display suggested keywords and estimates on them.
13) Submit Express Keyword Suggestion Tool
A keyword suggestion tool that uses data from wordtracker and keyword discovery
14) Google Trends
Google trends is great to use after you find some good keywords. The tool will display charts on the search volume that each keyword has. The bad thing is that they don't display charts for every keyword but it still helps a lot.
15) Yahoo Buzz
The same with google trends but this is using Yahoo's search engine for the results and it is categorized.
16) Google Zeitgeist
This is the newest tool from Google which displays top queries from the Google search engine on weekly basis.
17) Lycos week's top 50
The top searches from Lycos on weekly basis
18) Google Suggest
This is not really the best tool but it can help a little. While you type a search term the
19) SEObook Google Suggest Keyword Suggestion Scrapper Tool
This tool is using google suggest to display results, but it also give some extra information
20) Keyword Lizzard
Keyword Lizard is a FREE AdWords keyword tool that explodes your keyword lists by creating every possible permutation combination of keywords.
21) Ontology Finder
goRank's Ontology tool displays related search terms for the keyword you are using.
GoLexa displays several information on the sites that a search term returns. Great tool to monitor your competition.
23) Seochat keyword optimizer
With seochat's keyword optimizer you can optimize your list of keywords. This tool is more useful for cpc campaigns
24) Bulk Overture Keyword Tool
A tool that helps you do multiple queries to Overture.