Re: Coverity is decent
It is also available free to FOSS projects.
While there are numerous warning that can be ignored, the golden rule for all such code-profiling tools is to make sure you understand the nature of the warning before you fix it or ignore it.
Also worth a mention are some free (at least on Linux, maybe others?) memory checking tools like valgrind and the good old electric-fence library. While not checking your source code as such, they do help with detecting run-time memory errors such as double-free, leaks, etc.