Fess is very powerful and easily deployable Enterprise Search Server. You can install and run Fess quickly on any platforms, which have Java runtime environment. Fess is provided under Apache license.
Good open source project. It is easy to setup and get it running.
Pros
Fully open source and easy to setup. If you understand search, it doesn't take very long to set it up.
Cons
A little bit short on documentation. A lot of them is just a picture with descriptions, it needs to go much deeper. At least, it should give some sample values for the inputs and what it means.
