I really don't use SharePoint Designer all that much but on occasion I do find it really useful.  The case I am talking about today is using it to view and modify values in a site's property bag.  To do this, open SharePoint Designer and go to the Open Site menu.  When looking at the Open Site menu it isn't apparent at all that you can enter a URL there, but you can.  Enter the full path (i.e.: http://moss-server/MySiteCollection/MySite) to the site you want to look at then click Open.  Enter your credentials if necessary and then you should get a view of the site.  Ok, that part is pretty basic and you probably already knew how to do that.

To view the property bag.  Go to Site -> Site Settings.  Then click on the Parameters tab (not sure why they didn't just call it Property Bag).  On this tab, you can see the values of all of your custom property bag values (it doesn't show anything built-in).  If you need to, you can also modify and remove values from the property bag here.   Hopefully this tip is useful sometime in the future when you need to check the values of your property bag.  It sure beats the alternative of writing code to display it or firing up the debugger.