Here are some tools I use for estimating traffic potential for keywords:
The Google Adwords traffic estimation tool gives you an estimation of traffic to a particular keyword(s) or allows you to plug in a URL for a particular site to crawl and bring back suggestions.
The figures from Adwords are displayed in graphs and not actual numbers, but it gives you a good place to start you keyword research as it generates lots of suggestions. Keep in mind though that this tool is based on PPC traffic, not natural.
Keyword Discovery is by far my favourite tool out there. It allows you to plug in a keyword and outputs suggestions including estimated daily searches. Its data is compiled from 180 search engines.
If you had some money to invest I would recommend investing in the commercial version of this tool.
Word Tracker also allows gives you suggestions based on a seed keyword including daily estimates. Its figures are based on statistics from Meta Crawlers (such as dogpile.com and metacrawler.com) which account for 0.63% of all global searches). Word Tracker is a good tool, but if you had a choice between Wordtracker and Keyword Discovery I would go with Keyword Discovery (see above)
Wikipedia Traffic Estimation Tool
The Wikipedia shows up for more and more generic (1 word) keyword phrases than any other site out there. This tool, that estimates daily page views from the Wikipedia, allows you to get an idea of keyword potential (you need to account for 15-20% internal search traffic from within the Wiki)
This tool from Submit Express combines the results from both Keyword Discovery and Word Tracker in one interface, very handy and saves lots of valuable time!
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