Not a lot of people know about Google Analytics Filters. These filters are available in the account profiles and let you exclude or modify data.
Here are a few example where it can be interesting to use filters:
Be Careful! Filters do modify the data itself, so changes applied using filters are permanently done to the profile. If mistakes are made when setting these filters, removing them will later not bring back to data to its initial state. It is very important then to be very careful when setting filters. What is recommended then is to create a Master Profile, when no filters are applied, thus containing only the raw data. You will then always be able to refer to this Master profile in case of a problem.
Also note that goals are also set in the Google Analytics Profiles. You then have to make sure that the necessary goals are also created in your other profiles where they are needed.