A great question and one I will share the answer to here. First, why would you want to hide your plugins? Well think about security. If a hacker can see into your plugins folder and see what plugins you have installed a hacker can use their bag of tricks to deface or hack your site.
With Google, if they see automation plugins they do not like or auto-promotion tools they do not approve of, they can take action to penalize your site.
Personally, I like to keep peoples noses out of my setups
So here is what you can do to protect your wordpress plugins folder.
1. Put this in a .htaccess file inside your plugins folder
AuthName "Restricted Area" AuthType Basic AuthUserFile /home/.htpasswd AuthGroupFile /dev/null require valid-user
2. Stop Prying Eyes Eating Cookies...
This is another security tweak to stop people hacking your cookie file (it is possible)...
Put this in your main .htaccess file in the same folder as wp-config.php
RewriteEngine On RewriteBase / RewriteRule ^cookie.txt$ index.php [L]
WP Security Scan
WordPress › WP Security Scan « WordPress Plugins
WordPress › Secure WordPress « WordPress Plugins
WP Firewall 2
WordPress › WordPress Firewall 2 « WordPress Plugins
Hope that helps some people add a little extra protection to their wordpress blogs and keep out prying eyes.
All the best