<p>Are there any plugins/tools available to go through the classpath of an eclipse project (or workspace) and highlight any unused jars?</p>

ClassPathHelper
is a good start.
It automatically identifies orphan jars and much more.
The only limitation is with dependencies that are not defined in classes, e.g. in dependency injection framework configuration files.
You also have other options/complements, such as:
workingfrog "Relief"
, which relies on the ability to deal with real objects by examining their shape, size or relative place in space it gives a "physical" view on java packages, types and fields and their relationships, making them easier to handle.
Unnecessary Code Detector
: a eclipse PlugIn tool to find unnecessary (dead) public java code.