There are some tools provided by salesforce for this purpose. You can start with dataloader http://wiki.developerforce.com/page/Data_Loader which allows you to extract data from salesforce in to .csv format which can be uploaded to another database.
Another option is to use force.com API to create webservice connection and extract the data into the format you need (XML, JSON) and write another program that uses JDBC to connect to Oracle or mysql or any other database that you want to upload. I would start with the volume of data that you need to extract and based on the volume use the appropriate solution.