Wanna know the amount of Java jobs versus .Net jobs, or the growth of AJAX jobs? Google Trend may be able to help you a bit, but the result is not scoped for jobs only. Indeed.com has a bunch of cool tools for these kind of job data-mining.
For example, here is the big jump of AJAX jobs during the past year. I'm a little suspicious about this comparison between Java jobs vs .Net jobs.
The website states that "Indeed.com searches millions of jobs from thousands of job sites. This job trends graph shows the percentage of jobs we find that contain your search terms." Not sure how good their sampling is.
Another highlight is Job Postings Per Capita. I'm surprised to see NYC ranks only 40th, after Kansas City and Columbus, OH, both with much smaller population size.