Version

How to find the version of the server.

Note: Version and Revision in the following files mean the same thing. (I have no idea why they switched words)
So version=2096 means the revision is 2096.

builddate is not the revision number, it is the date and time that your copy of a revision was compiled.
(You could take revision 1 and compile it, giving it todays date, when in fact it is really years old)

Server

Open your /gameserver folder. Open the /config folder. Open the l2j-version.properties file.
The info should be in there.

    	version=777
    	builddate=20070504_0111 
      detailed info:
          http://l2jserver.com/svn/trunk/L2_GameServer_It
          http://l2jserver.com/svn
          2007-05-03T19:04:16.979804Z
          777
          Sami

If it only says ${l2j-revision}, then you may want to install the subversion commandline utility.
http://subversion.tigris.org/servlets/ProjectDocumentList?folderID=91
The subversion commandline is optional. It is not needed to run the server.

Datapack

Open your /config folder. Open the l2jdp-version.properties.

As of early May 2007, the version can be found in 2 places in both files mentioned above:

Eclipse

In Eclipse, in the Navigator window, Right click L2_GameServer > Properties >Subversion

I get this error when compiling with Eclipse:

[exec] Execute failed: java.io.IOException: CreateProcess: svnversion -n . error=2

The reason for this non critical error is that you do not have subversion commandline installed. You have the subversion plugin for Eclipse.

Installing the commandline version of Subversion

If you would like the commandline subversion you can get it here

Again, this is not needed. Only optional.
Server will compile and run just fine without commandline subversion installed.

Learning more about Subversion

Subversion is a powerful version control software, here is the site and here their cool book. Take a look, you could be the next one to commit here :)