FitNesse is the acceptance test wiki.

FitNesse is an easy to use wiki web server

The wiki syntax in FitNesse is minimalistic and helps you concentrate on the content. It has a rich-text editor for those not really into the wiki markup. The rich-text editor has extra features to support table creation and modification, something you'll love when creating test tables in FitNesse.

Easy to set up: just download the application (a Java jar file) and start it.

FitNesse comes as an all-on-one package. Starting FitNesse is as simple as starting the Java jar file. Wiki pages are stored as files, so there is no hassle with databases. FitNesse can be run on your machine or on a server. The website is running FitNesse.

Specifications/requirements can be used as test input

Since FitNesse is a wiki, it's not in the way when you reason about requirements for your (software) project. FitNesse allows you to validate those requirements with the actual software implementation. If your project is written in Java, it's a piece of cake. However FitNesse plays well with every language and environment.

The major languages are supported

Yes! Whether your project is written in Java, Python or C#, FitNesse can be hooked in and you can automate the test process. See the PlugIns page for impementations.