Rand recently posted a comparison of traffic volumes and competitive research data for various SEO Blogs.
Here is a brief overview of a few of the other free competitive anaysis tools on the market. All I ever have to do to realize how Alexa is inaccurately skewed toward marketing and webmaster resources is look at my own graph. There is no chance in ___ that this is one of the top 1,000 sites on the web.
Recently there have been a couple launches of services which compete with Alexa, and appear to have quite a bit less webmaster skew to them.
Compete.com gives a snapshot of your site which includes average pageviews per visitor and average time on site.
Quantcast.com gives a snapshot of your site which includes demographic information (they think 90% of the readers here are guys), and breaks your site visitors and traffic volumes down into passers-by, regulars, and addicts. Sites which have highly engaged visitors are typically going to be much harder to compete against than sites which are entirely reliant on search.
There are also a number of tools which show you what keywords competitors rank for,
Spyfoo shows top ranking keywords, competing advertisers and organic competitors.
URL Trends shows historical link trends and some of the keywords a website ranks for.
I honestly do not use any of these tools much yet, but find them interesting. Do you find any of them or any similar tools useful for search marketing category analysis?