A popular commercial IDE with a free community edition.
Ranked in these QuestionsQuestion Ranking
Pro Fast and smart contextual assistance
Uses a fast indexing technique to provide contextual hints (auto-completion, available object members, import suggestions).
On-the-fly code analysis to detect errors and propose refactorization.
Pro Stable and robust
Nothing like Eclipse, you don't have to restart projects from start just because they were corrupted by the IDE!
Also loads much faster than Eclipse.
Pro Lots of plugins
Many plugins are available for almost any task a developer may need to cover. Plugins are developed by Jetbrains themselves or by 3rd parties through the SDK available for writing them.
Pro Smart refactorings
IDEA places an emphasis in safe refactoring, offering a variety of features to make this possible for a variety of languages.
These features include safe delete, type migration and replacing method code duplicates.
Pro Intuitive and slick UI
Pro Android support, JavaEE support, ...
A very complete development environment support.
Pro Free version available
There is a free community edition (open source) and an ultimate edition, which you can compare here.
The ultimate edition is available for free for one year for students but must be registered through an .edu e-mail account.
Pro Support for many languages
IntelliJ supports many languages besides Java, some of these are: golang, Scala, Clojure, Groovy, Bash, etc...
Pro Clear and detailed documentation
The documentation is exhaustive, easy to navigate and clearly worded.
Con Built with closed source components
The version with full features is not opensource. Parts of the code are under apache licence though.
Con Bugs not solved
They are more interested in adding new features or issuing new versions than solving bugs.