This plug-in allows you to deploy your working eclipse 'project' in one single jar with just one easy step. It finds all the dependencies your project currently depends on --Jars, External-Jars, User-Libraries, System-Libraries, Classes-Folders and Project-Exports are taken into account.
In the export wizard you can also select the main class for the jar being exported.
As far as I have used, it performs very well, though the project is still in beta stage. It works joining all the dependency clases and resources into one single jar, very handy for some situations where you develop some swing interfaces, or simple demons or small things like that. Give it a try to FatJar Eclipse Plug-in, it really worth !
Of course, this is not a replacement for tools such Ant or Maven, because this is a complete different approach for buildings. Do not use it for very big projects, but for small to medium sized applications I think will give you a lot less headaches than dealing with formal building tools, saving you time and effort while providing what you just need: a working Jar :)