I have seen many products bundeled with jdk, I wonder if there is a way where one can install JDK by simply unzipping contents to a directory. so, there is no icon created in add/remove programs, no registry entries etc.
Also in this case: How can we configure java plugin for browsers? And how can we configure settings as seen via control panel entry for java?
Really, no, at least if you want to use it from Windows and not from, say, cygwin. Windows depends too much on the registry; you could simulate the registry updates necessary, but software that moves the files to the right place and updates the registry is called "an installer"; you'd just be reinventing the installer.
I believe this at least used to be feasible to a limited extent with earlier versions of Java - I don't know if it still is.
Most of the registry entries are used for things like browser plugins, as you mentioned in the question. If you just want to be able to run Java (e.g. from a batch file), that's one thing - but really installing it is a different matter. Even if you're just wanting to run it, you'll need to be careful to always explicitly use the one you intend, rather than using the installed one accidentally for part of your application.
What's your actual use case? Do you actually need a browser plugin? What aspect of the configuration are you interested in? A lot of the control panel configuration is to do with updates and browser integration. Many other aspects can be controlled using command line options to set specific system properties.
If you just want to provide a JVM with your application is fine, but more than that I would not recommend.
Yes, you can create a zipped up JDK, unzip it on the target machine and run java, javac, etc. from that directory to your heart's content.
The easiest way to create such a zip file is to install the JDK on a machine and then zip up the contents of the JDK directory. We do this in certain circumstances where we need to control exactly what configuration of Java will be used in our deployments. In that case, our scripts just point JAVA_HOME (or the equivalent) to our internally bundled JDK instead of relying on a sysadmin to install exactly what we need prior to arrival.
In terms of integrating with the browsers, that can be a bit more problematic. The short answer is no, you can't integrate directly with the browser without some sort of installer.
You could use SysInternals RegMon and FileMon (now owned and dist by MS) to see exactly what is modified by the Java installer. I believe there will be a number of reg entries that you will want to create. Products like WISE installer, for example, also use this sort of approach under the hood to repackage product installations (e.g. as MSI).
Please be careful since there are also some dynamic decisions made by the installer which may affect what gets installed (e.g. on XP v.s. W2K3 server). I was bitten by this once regarding installed codepages. I do not recall the precise details, but the effect was that a codepage file was missing in my embedded JRE + JDK (legally redistributable portion only). This caused a very bizarre and seemingly nonsensical runtime error in my code. It goes without saying that the same applies to Server v.s. client JVM DLLs.