How to add field from related Account to Contact and Opportunity

[Cooper007]

I've created a custom field in both Leads and Accounts. When I convert a Lead, the field data is placed in the Account record. Now I'd like to display the contents of that field on the Contacts and Opportunities that are related to a given Account. I don't care if the field is editable or not on the Contact and Opportunity records.

I was thinking this would be simple to do, but I can't figure it out - any tips?