If you mean, do I know a website that will assemble a JAR, I don't. However it's not too much trouble to create such an app and deploy one on a website.
For a
J2EE server, the java.util jar classes can be used. For non-java servers such as PHP or .Net, any app that can create a ZIPfile works, since a JAR is just a ZIP file with a META-INF/MANIFEST.MF file in it.
However, uploading a bunch of files to the server is such a pain that you might as well zip/jar them before even uploading, in which case what benefit would a server JAR service be? All you'd really need would be a server that downloaded JARs, and almost any web server can do that.
The secret of how to be miserable is to constantly expect things are going to happen the way that they are "supposed" to happen.
You can have faith, which carries the understanding that you may be disappointed. Then there's being a willfully-blind idiot, which virtually guarantees it.