Edward Dale has grepped through the WordPress plugins repository to find the most frequently-used filters and action hooks.
In case you didn’t know, the filters and action hooks API is the glory of WordPress: it’s what makes WordPress so easily modified and extended. Seeing which are the most often used should say something about what plugins in general are trying to do.
I think the most interesting result of Dale’s survey is that so few are used frequently. Once you get past the action hooks that have to do with plugin/widget initialization and the filters of post text, few remaining are statistically significant. That suggests to me that most plugin developers are under-utilizing WordPress’s full potential.
See Dale’s results here.