Expression Engine CMS has free and paid options for personal sites and a higher priced paid license for commercial use, so you might need to check into whether there's a developer's license needed for multiple sites.
If you are considering multiple sites then you should definitely take a look at EE ExpressionEngine. It has a Multi Site Module which is proving popular. EE is about to be upgraded to version 2.0 and there is great excitement among its fans. Opps corrected the name. No gap between Expression and Engine.
[edited by: Durham_e at 10:03 pm (utc) on Sep. 4, 2008]