Tools are a-plenty, for many languages, but here is a good starter for your favourite open platform: Byecycle
Other tools do more, but are also more complicated and hard to interpret. If you want a gentle introduction, start with this little visual tool.
It can help you find dependency cycles
It can help you spot classes that do too much
It simply gives you a 30.000ft view of how your code looks, something which is difficult to get from text files on even a medium sized code base.
