One thing that should not be left in code as comments is old code.  There are many free tools that left you version your code.  Subversion is a great tool when used with TortoiseSVN.  Leaving commented code scattered all over will cause you to second guess yourself, all distraction to the real code, and is just bad practice.

If you have a versioning solution, take time to go back through your code and clean things up.  You may find that you can remove lines and leave real comments that are far more knowledgeable than having to remember why you commented out the old code in the first place.