Google just launched Google Trends and I already like it. You can visualise search volume for up to 5 queries over time. You can sort results by years, cities, countries and languages. You can do comparative searches. See my example on comparing searches for French cars in France. It seems to match with market shares.
The bonus is that Google maps news stories (from its Google News service) on the graph so you can see correlations between news and searches. A news volume graph is also displayed underneath the search volume graph.
Google explains that the results are extrapolation from a sample, therefore cannot be entireley accurate.
Considering that 81% of consumers aged 30 to 64 years old use the web to research products or services (Pew Internet Survey 2005), I can see Google trends being used for "share of searches" (share of voice) over time to assess popularity of brands and products or look at the impact of marketing campaigns. It is fair to assume that more people should search for a particular brand after an awareness campaign is being aired (or PR through media coverage on Google News). You could roughly compare how this trend matches sales volume.