Refer to my answer on v7n. However, another one popped into mind: statbrain.com and quantcast. They are all trend estimates and remember they are really estimating private data.
I found a tool that can do that I think its compete.com I'm not sure I found that a long time ago not sure its still live.
you might also try TrafficEstimate.. well just lile any other tool it might shows estimate results.. but it can help..
Well all above traffic estimated sites are nice but i don't think is there any free website that give you accurate data for how much competition for your website by particular keyword.
IBP is a good application which provides a complete detail and customized report. I think the question is regarding the keywords competition. If so google adword keyword tool is a good web application provided by google.
Beside Alexa. There are various tools. You could try websitetrafficspy, webtraffic24, compete.com (you need to create a free account). But keep in mind that the majority of these tools track unique visitors and process data differently. There can be huge variation between data shown. It is not really worth worrying about your competitors traffic. Just concentrate on your own traffic and ranking.