SQL 2005 Online Index (Only Available in Enterprise Edition)

Index operations can now be performed online; users can still access the table data and use other indexes on the table while one index is being created, altered, or dropped. In SQL Server 2005 EE , you can create, rebuild, or drop indexes online. The ONLINE option allows concurrent user access to the underlying table or clustered index data and any associated nonclustered indexes during these index operations.
Eg.
USE AdventureWorks;
GO
ALTER INDEX ALL ON Production.Product REBUILD WITH (ONLINE = ON);