How do I find all the available SQL Servers on my network?

This script will only find those servers (and instances) that are broadcasting their existence, and requires execute permissions on XP_CMDShell. 
 

CREATE PROCEDURE dbo.ListLocalServers  AS  BEGIN      SET NOCOUNT ON        CREATE TABLE #servers(sname VARCHAR(255))        INSERT #servers EXEC master..XP_CMDShell 'OSQL -L'      -- play with ISQL -L too, results differ slightly        DELETE #servers WHERE sname='Servers:'        SELECT LTRIM(sname) FROM #servers WHERE sname != 'NULL'        DROP TABLE #servers  END

 
So from ASP, you could do this: 
 

<%      set conn = CreateObject("ADODB.Connection")      conn.open "<connection string>"        set rs = conn.execute("EXEC ListLocalServers")        do while not rs.eof          response.write rs(0) & "<br>"          rs.movenext      loop        rs.close      set rs = nothing  %>

 
(Note that this won't tell you which port SQL Server is listening on.) 
 
Another option is to use SQL-DMO's .ListAvailableSQLServers() method. Rather than pretend to be a SQL-DMO expert, I'll simply point you to this post. 
 
Gert has some great information as well: 
 
    Enumerate SQL Servers using OLE-DB 
 
    Enumerate SQL Servers using ODBC 
 
    Enumerate SQL Servers using LAN manager