I have been playing around with the new Microsoft Assessment and Planning (MAP) Toolkit 5.0 Beta 2 tool for the last couple of days, and so far I am pretty happy with it. It lets you scan machines using Active Directory and WMI, so you can inventory your current hardware, operating systems, and applications, including SQL Server.
It can then generate a number of spreadsheets and Word documents that give you a pretty good summary and detailed view of your environment. The tool is pretty useful for finding “rogue” instances of SQL Server (assuming you have Windows or SQL Server credentials with enough rights to access the machine or login to SQL Server), and instances of SQL Server that need to have Service Packs or Cumulative Updates applied.
One problem I do have with the tool is that it insists on installing a dedicated instance of SQL Server 2008 Express Edition on your workstation, even if you already have an instance of SQL Server 2008 or 2008 R2 installed there. The tool is free, and I have discovered a number of SQL Server instances that I was previously unaware of. Another similar, but more focused tool is Quest Discovery Wizard for SQL Server Beta, which is also free. Both of these are pretty handy to have around as a DBA.