As for MonoDevelop, it includes a Boo binding, written by Peter Johanson, leveraging the parser code written by Daniel Grunwald for the SharpDevelop binding. It includes Boo project creation/editing/compiling, as well as an interactive shell with Gtk# integration. See the monodevelop page for more information on installing it.
I see references on the site for .NET 1.1 and .NET 2.0, does Boo
support .NET 3.0?
.NET 3.0 is actually just an update to the framework and not to the
CLR or any of the "official" languages. As such, it should be
supported by any .NET-2.0-supporting language, such as Boo.
Also, Boo release .78 is the last version of Boo that will support .NET 1.1.
"What's a good way to get started with Boo (editors/IDEs)?"
Fire up a console and check out booish – out - a built-in editor to check out the basics. Then grab a copy of Sharpdevelop or monodevelop ease into to dive into developing with Boo.
"What do people use for building 'real' Boo applications?"
On Windows, Sharpdevelop is the most robust and stable IDE for developing BOO applications. Linux and Mac users develop with their favorite text editor. The monodevelop team are hard at work developing a more professional development environment that will support Boo along with other .NET languages.