SpotBugs is the spiritual successor of FindBugs, carrying on from the point where it left off with support of its community.
More information at the official website. A lot of things can still be found at the old FindBugs website.
SpotBugs is built using Gradle. The recommended way to obtain it is to simply run the gradlew (or gradlew.bat) wrapper, which will automatically download and run the correct version as needed (using the settings in gradle/wrapper/gradle-wrapper.properties).
To see a list of build options, run gradle tasks (or gradlew tasks). The build task will perform a full build and test.
To build the FindBugs plugin for Eclipse, you'll need to create the file eclipsePlugin/local.properties, containing a property eclipsePlugin.dir that points to an Eclipse installation's plugin directory (see .travis.yml for an example), then run the build.