We in our organizationa are trying to implement a source code quality management tool. Sonar is one such tool we have come across, and it's quite full of features and is phenomenal. We want to compare it with it's peers, if there are any, before we actually implement it.
Are there any good contenders to Sonar's capabilities and features?
Since you are just starting out in that field simply go with sonar and re-evaluate in 1 year.
if you are concerned only about quality of code, then Sonar is a great tool and it is a way to go.
Though, I would recommend you to also look at the quality from a broader perspective. For example your customer would not care a lot about your code quality and instead would evaluate quality as number of bugs he encounters in your product releases. So in addition to code, you may analyse your defects, quality of your development process and probably many other aspects.
You may take a look at http://www.programeter.com, if you are interested in quality of the product as a whole, and not just the code quality.
You don't indicate your platform. If you're an Eclipse shop, there's CodePro Analytix.