Here'a a couple of what sounds like a decent tool I found in 10 minutes using my assistant DBA, Google. 😛

I like this first one the most because I can call Powershell from xp_CmdShell (captures the result set directly) or just run Powershell separately and pipe the output to an importable file.

http://www.mssqltips.com/sqlservertip/2013/find-sql-server-instances-across-your-network-using-windows-powershell/

Here's another that goes above and beyond.

http://www.straightpathsql.com/archives/2009/02/what-sql-instances-are-installed-on-my-network/

http://www.microsoft.com/en-us/download/details.aspx?id=7826

MAP (Microsoft Assessment and Planning Toolkit) (from the links above) supposedly makes a spreadsheet and a word document with all the goodies you might care to lookup.