As you may be aware, you can add custom fields to all modules and custom modules in CRM. What I want to discuss here are some of the fields you might forget to use or considerations for using certain fields.
First, always remember that if you are making a field in Lead, you may want to check the button to create the field in Contact, Account and/or Deal as well, based on your requirements. If you forget to do that, either delete the field completely (its a 2 step process) and add it again or use Lead Conversion Mapping to map the fields.
Go to Setup -> Customization -> Modules and Fields -> Layouts and then you will come to the area where you can add custom fields. The fields available are as follows: