Adobe has just released a free public beta for Lightroom 5.
Adobe's Lightroom blog has the announcement where you can see the list of new features, and also a list of resources (essentially other people writing about the release). In particular, Victoria Bampton's What's New post is always a good reference.
(Unfortunately, Adobe mistakenly schedule the release during my cherry-blossom season, so I've had no time to write up anything about it myself.)
The latest versions of my Lightroom plugins should work in Lightrooom 5.. Be sure to upgrade to the latest version of each plugin to work in Lr5
This is the most exciting Lightroom upgrade for me in many years, but perhaps surprisingly, my excitement is not due to any new image-editing feature (though the new healing brush is amazing), but because the new “Smart Previews” feature will allow me to have my entire multi-Terabyte photo library on my laptop.