Nowadays I deal with some SQL Server consolidation engagements. Many customers have hundreds of SQL Server instances with many different versions (7.0, 2000 SPx, 2005 SPx) and they have some difficulties managing these due to the over-distribution fact.
The main idea is to consolidate these SQL Server instances to central servers so that
Compared to OS Virtualisation I believe application consolidation (Database, Web application etc) have many advantages (maybe I should write another post for this as well)
We use some tools to make our lives easier