Omega is a great example of a theme framework for Drupal. We've previously covered Omega in a detailed tutorial which explains how to get started.
Omega is perhaps the most popular framework currently. It was the first mobile-first, responsive base theme to date, paving new ground in the Drupal community. It was powered by a modified 960 grid implementation. This is the theme used by Acquia.com and several other major players.
However, using Omega is not easy for beginners. If you install Omega and make it into your default theme, this is what you'll see:
Respond works in three size modes: mobile, fluid and 1200px maximum. Images scale using the fluid images method. This theme is meant to be a proof of concept on some CSS3 features in Drupal as an Omega subtheme. You can download it from http://drupal.org/project/respond.
Alphorn is a dark theme, available in 3 options: default, grey and green. It hasthree menu options available, a flexible layout and different combinations for you to choose. You can download it from http://drupal.org/project/alphorn.
Tej is a colorable sub-theme. it uses the color module (like Bartik theme), which allows you to modify the color of the theme to your liking. There are many preset color Schemes and you can generate your own by using the color picker on the theme settings page. You can download it from http://drupal.org/project/tej.