Is there something like Snoop (WPF) or FireBug (ASP.NET) for Windows Forms?

Hawkeye (seems moved to github) can do that and more. You can see all the properties, fields, events and methods of a control. You can also edit the properties during runtime.

I recommend WinSpy++ because its much easier to get hold of then the original Spy++ which comes as part of the Windows SDK.

Just like Spy++ this only works at the Win32 level, of which WinForms is a thin/medium wrapper.

I havn't tried it, but there is a tool called ManagedSpy (Article) that might be helpful.

Spy++ allow you to see Window settings and messages, but that works at native (Win32) level, and will not show .NET information.