The one feature you will most certainly find in a commercial static analysis tool (and that you will not find easily in a freeware analysis tool, at least in 2008, at the time of the OP) is
Reporting: Measures software quality trends over time
As explained in this question about code metrics, any static code analysis in itself in not always meaningful, because you could have:
- too many "defects" to fix
- too many categories of defect reported
You need the ability to do some triage, and you need to check if a particular defect is occurring less and less over time or not, in order to help you prioritize what to fix.
This is especially true on legacy project with thousands of classes: you do not fix defect on many files just like that, without having a good reason. That reason can be deduced from a good reporting and trend analysis you will not find with freeware tools.
Update: from 2012 (4 years later), Sonar (Now in 2018 named "SonarQube") "Historical Information" (aka "Time Machine") in its 4.x and 5.x series.
Note those project dashboards were dropped in SonarQube 6.1 (Sept. 2016): see this thread.
Those dashboard would need to be re-created manually through a custom page.
SonarQube 6.5 restores a bit of those dashboards with the Activity page, which gets (several predefined and one customisable) charts to display the evolution of a project.
