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

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.

My favorite (for ease of use) is our modified wfSpy utility.

It is available at my blog: http://www.cplotts.com/2009/10/28/an-ode-to-wfspy/

The original utility is from CodeProject ... but that version doesn't have the ability to drag the mouse on top of a control in your application.

Hawkeye (already mentioned above) is a favorite of mine as well ... more powerful than wfSpy ... but definitely harder to use. wfSpy is great for quick and simple spying (and of course editing of properties on the fly).

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.