Adobe have released Lightroom 2 including many new feature:
Local adjustment brush
Enhanced organizational tools
Volume management
Extensible architecture
Multiple monitor support
Flexible print package functionality
Streamlined Photoshop CS3 integration
Enhanced output sharpening
64-bit support for Windows® and Mac OS
There's more detail
here and in the press release
here.
I know a lot of people have been waiting for multiple monitor support but to me the most important new features are the improvements to local adjustments and, in beta form, support for camera specific profiles available to download
here. The custom profiles are designed to emulate the manufacturer’s own conversions and picture styles.
It's also well worth reading
this article for an extremely detailed list of the new features.