Today the Milestone 3 of the VisualVM tool has been released. It contains several UI and features improvements, also performance and memory management have been improved. The build can be downloaded from https://visualvm.dev.java.net.
Sources of the tool are available from a CVS server, Milestone 3 sources are on visualvm_m3 branch. There is a Build Guide available to provide basic information on how to build the tool from sources. A zipped JavaDoc snapshot for Milestone 3 and API Entrypoints document are available for plugin developers - just keep in mind that the API is experimental and thus subject to change for upcoming milestones.