Using the data dump, I've put together some tag trend graphs that show the rate of tagged questions over time. Below are some samples, there are many more on the linked page. I can update the page with requests for specific tag groups if you're interested.
Update: There's a new interactive tag trend graph page where you can select any set of tags to compare. Also, the graphs are improved and have a labeled X axis.
Distributed Source Control (git, mercurial, bazaar)
Meta (subjective, not-programming-related, polls, belongs-on-serverfault, stackoverflow)