Some users require a theme that has many configurable options while others need a blank canvas they can use as a starter theme. Some users would like to re-color their theme using the Color module and some would like a dedicated administration theme. Sites in Right to Left languages such as Hebrew and Arabic require a theme that specifically supports RTL.

Drupal themes can serve all these purposes and I have picked out a few themes from each category as examples. Please feel free to add to this or suggest changes.

Please note that there are many more themes available for download.

End User Themes
This type of theme typically includes many "theme settings", which allows an end user (not a developer) to have a customised version of a theme, rather than a completely unique theme. Examples of this type of theme include ABC, Tapestry, Black Hole, Artists C01, Zero Point, Yahoo Pure.CSS Framework and Acquia Marina.
Colorable Themes
Colorable themes can be recoloured using the color wheel in your theme settings and usually have many pre-set color schemes to choose from. Typically these themes have a high level of design and you can have a very nice theme simply by choosing a different color scheme. Examples include Bartik and Garland in Drupal core, Pixture Reloaded and Twilight.
Starter Themes
These themes are designed with the intention of being customised. Typically they have little or no style and offer tools and features aimed at making it easy to develop your own unique Drupal theme or subtheme. Examples include Zen, Yahoo Pure.CSS Framework, Fusion, Ninesixty, Nucleus, Genesis, Zero Point-Themer, ABC, Foundation and Black Hole-Themer.
Admin Themes
These themes are all about theming the administration section of your Drupal site. Some examples are Rubik, Root Candy, Activesite, Fruity, Stanley, Blossom, Cube and OM Admin Theme.
HTML5 Themes
Themes using HTML5. Examples are ABC, HTML5 Base, Boron, Black Hole, Groundwork, Adaptive Theme, Yahoo Pure.CSS Framework and Zero Point.
Responsive (mobile first) Themes
These themes are build to provide an optimal viewing experience — easy reading and navigation with a minimum of resizing, panning, and scrolling — across a wide range of devices (from desktop computer monitors to mobile phones). Examples include Zen, Yahoo Pure.CSS Framework, AdaptiveTheme, Fusion and Zero Point.
RTL Ready Themes
These themes have features that fully support RTL, or right to left languages. Some examples are ABC, Black Hole, Colorfulness, Garland, Interactive Media and Zero Point. Yahoo Pure.CSS Framework, Tendu and Zen which are starter themes, offer good out-of-the-box support for RTL also.