The Eclipse-class star dreadnought is an extremely large warship, of which only two were ever built. Both were constructed to serve as Darth Sidious's personal flagship. They're 17 kilometers long: a bit shorter than the Executor-class ships, but they were much wider. Considering the initial statements about the size of the Executors, it's fairly obvious that the Eclipse ships were meant to dwarf them.
Eclipses were among the most powerful warships to appear in the Star Wars EU. They had enough firepower to engage entire Rebel fleets at a time, and their armor and shields were so strong that they wouldn't need to think twice before ramming other ships (one these rammed through the Galaxy Gun without taking any damage).
In addition to their standard weapons and huge number of starfighters, they also possessed gravity well projectors to keep enemies from escaping inter hyperspace. The crown jewel of their features, however, was their own superlaser. Though not in the same league as the super-lasers possessed by the Death Stars, they were still powerful enough to sear continents and/or crack open a planet's crust. The resulting heat and superheated chunks of debris being ejected would likely kill everything on the planet.