OpenSourcery

The Drupal Views module has powerful theming abilities. CSS classes abound, allowing you to accomplish a great deal with stylesheets alone. If that's not enough for you, custom phptemplate files can be added to your theme. The new Views 2 will even helpfully suggest a list of possible template files (under Basic Settings -> Theme: Information).

Wouldn't it be nice, though, if you could have custom markup independent of the theme? Or perhaps you'd like to apply your custom styles more selectively to just certain views or view displays?
With hook_views_plugins(), you can.