Included in this category is stuff that most people do remember- paying for the server hosting, any software licensing costs, and paying for the URL (although you would be surprised at the number of clients who forget that they have to renew their URL payments or risk losing it). 

However, in addition to these costs, there are additional maintenance tasks that need to be performed, including:

• Offsite functioning backups and archives of the code and data in the database (essential for recovery of your site if for some reason the server is destroyed).
  
  
• Security patches to the server and to the software (new holes and vulnerabilities are found all the time. Get worried if your server or site NEVER needs patching).
  
  
• Security log tracking (to identify any issues AS SOON AS a major incident occurs).
  
  
• Responding to software issues, bugs, data errors, etc (No software is "bug free". In data entry heavy sites,  a "bug" can be caused by bad data being entered. In some cases, the bug is actually a mismatch between the expectations and the design).
  
  
• Responsiveness to emerging needs (such as a security incident, troubleshooting a site issue, resetting a password or offering technical support to an administrative user, and even making sure there are up to date copies of the site administration details.

Production Maintenance: Content » « Production Maintenance: Overview