WebP is a image-format - developed by Google - aimed to reduce the size of images dramatically. While still in development, WebP-images already allow you to reduce the size of JPG or PNG images. Unfortunately, the WebP image-format is at this moment only supported by a small number of browsers (Chrome, Firefox with extra plugin).
Note that you will need to install the WebP binaries on the webserver. If you don't have root-access or shell-access, you probably need to ask your system administrator to install WebP for you.
Less CSS and JS files on your pages increases your page speed. This extension allows you to merge files together, so that less HTTP requests are required.
Hosting images, stylesheets or files on a remote location, might save bandwidth - thus increase performance. This plugin implements a static server in Joomla!.
Yireo Template Helper
Every Joomla! template needs PHP-code, but using the Joomla! Framework can be hard. This library makes life a bit easier for template-developers.