Woodstox, high impact factor & being #32 on Top Open Source Java libs list
Another interesting data point, this time from analysing Maven Dependency paths: "Most Referenced" list. Looks like Woodstox is quite widely used by projects that use or at least declare their dependencies using Maven: I assume magic number 1838 (which gives rank #32) could mean number of other projects depending on Woodstox. Not too shabby for an xml parser. Getting on the first result page is quite remarkable; especially considering that Woodstox ranks higher than many other worthy Java open source libraries like XStream, Hibernate, Quartz, Xalan and Velocity. And only slightly (by about 50% :-) ) trailing such ubiquitous thingy as Spring.
Although this is just one of way of estimating popularity of various (Java) OS libs, it is still interesting, because it has similarities to how scientific articles are ranked (impact factor; although here weights are uniform). And also since it could lend itself to Google PageRank style extensions as well... let's see.