Callisto is a simulaneous release of 10 related Eclipse projects that cover pretty much everything you would ever desire to do within your IDE.
The list includes the Eclipse platform itself, plus a whole collection of other major plugins, covering: modelling, testing, J2EE development, profiling and visual development (UI).
Timothy O’Brien has an article on the O’Reilly site giving an overview of the currently available Callisto release covering all these features plus more.
I wonder if this is the reaction to the increasing popularity of the Netbeans IDE, which has been picking up serious momentum over the last few months. The major difference I see with the Netbeans IDE is that it includes support for all the common types of development and tools a typical developer needs, without having to mess around with downloading and installing plugins like in the Eclipse world (think WTP). Callisto seems to take this to the extreme though – they are including every possible major plug-in known to man (well, almost).