Finding out who has modified a Stored Procedure on SQL Server?

You can look at the default trace and figure this out easily. Try the examples from Jonathan Kehayias.

http://sqlblog.com/blogs/jonathan_kehayias/archive/2009/05/27/pass-dba-sig-default-trace-presentation-files.aspx

This is to show as an example and you need to look at the ALTER OBJECT event type.

http://sqlblog.com/blogs/jonathan_kehayias/archive/2009/09/29/what-session-created-that-object-in-tempdb.aspx

EDIT:

Code is courtesy of Jonathan Kehayias.

DECLARE @filename VARCHAR(255) 
SELECT @FileName = SUBSTRING(path, 0, LEN(path)-CHARINDEX('\', REVERSE(path))+1) + '\Log.trc'  
FROM sys.traces   
WHERE is_default = 1;  

SELECT gt.HostName, 
       gt.ApplicationName, 
       gt.NTUserName, 
       gt.NTDomainName, 
       gt.LoginName, 
       gt.SPID, 
       gt.EventClass, 
       te.Name AS EventName,
       gt.EventSubClass,      
       gt.TEXTData, 
       gt.StartTime, 
       gt.EndTime, 
       gt.ObjectName, 
       gt.DatabaseName, 
       gt.FileName, 
       gt.IsSystem
FROM [fn_trace_gettable](@filename, DEFAULT) gt 
JOIN sys.trace_events te ON gt.EventClass = te.trace_event_id 
WHERE EventClass in (164) --AND gt.EventSubClass = 2
ORDER BY StartTime DESC;