The largest Java IDE and tools ecosystem out there right now.
Ranked in these QuestionsQuestion Ranking
Pro Large selection of plugins
Eclipse has a large and active community, which has resulted in a wide variety of plugins.
Pro Good refactoring tools
Pro Highly customizable
Thanks to the large variety of plugins and various configuration options, Eclipse is very customizable.
Pro Free and open source
Eclipse is an open source project and free to use.
Pro Fast compiler
Eclipse uses a custom compiler (which can also be used outside of Eclipse), which is often faster than the normal Java Compiler, especially for incremental compilation.
Pro Multiple languages - one IDE
Pro Good font rendering
Because Eclipse is based on SWT, it uses the native font rendering and thus looks better than other IDEs on some Linux systems, where the Java font rendering is not optimal.
Pro Great debugger
Shows threads, concurrency locks, and conditional breakpoints.
Con Plugins can be unstable
Though there are plenty of plugins to choose from, they aren't always reliable. Some aren't maintained, bug fixes can be slow, and you may need to download plugins from multiple sources.
Con UI can be confusing
There's an overly abundant presence of menus, this forces you to constantly click around the different menu structures; foreign ideas, like Views and Perspectives; strange menu choices, like configure settings located in Windows menu->Preferences.
Con Poor language support via plugins
Eclipse supports other languages with a huge amount of plugins. Many languages have their own distribution, but multi-language is hard to exist in one project. Like Scala, there is no official support from Eclipse for this language. If Eclipse gets an update, languages such as these will not.
Con Lack of plugins with good user interfaces
Many Eclipse plugins are extremely confusing, with UIs that are even less consistent than Eclipse itself.