If you're anything like me, you'll still be getting the hang of String.Format ever since sprintf was removed from our toolbag. So, here's a tool to interactively play with a format specifier to see how it formats a value that's been parsed in each of the common types.

And here's the link to the often-hard to find MSDN format specifier references:

Standard numbers
Custom numbers
Standard datetimes
Custom datetimes

Here's the exe:
stringformat-exe

Here's the source:
stringformat-source