Many shops are considering consolidating their SQL Server Instances onto fewer boxes, as I've blogged about before. There are lots of ways to think about consolidation, from placing more databases on one Instance (Database Stacking), putting multiple Instances on one server (Instance Stacking), and using a Hypervisor (Virtualization). But before you do any of that, you should profile the servers and Instances you currently have so that you know what applications are candidates for consolidation.
So how do you do that? Well, you could learn lots of counters, objects, dynamic management views and so on, or you could just download the latest Microsoft Assessment and Planning Tool and run that. Here's the link for that, and here's an article I did for the product.