Using the JDK, jar -tf will list the files in the jar. javap will give you more details from a particular class file.
I usually open them with 7-Zip… It allows at least to see packages and classes and resources.
Should I need to see methods or fields, I would use Jad but of course, it is better to rely on (good) JavaDoc…
Now, somewhere on SO was mentioned some Eclipse plug-ins, to find in which jar file a class is located, perhaps they can do more (ie. what you requested).
[EDIT] Reference to SO thread. Not what is asked, but somehow related, thus useful: Java: How do I know which jar file to use given a class name?