Most rating widgets I have come across show stars in half or whole. This is usually implemented using images corresponding to half, full or empty stars. The user hovers over the widget to set the rating, again between whole and half stars.
However, sites like IMDB and Orkut have more granular ratings, showing partial stars. The implementation was interesting and in a way, more efficient. Doing it seems to be simpler than the older way of showing ratings.
Two images that look something like and are used to create the effect. The second image (or empty stars) is used as the background of a DIV. Inside this DIV, is placed another DIV, whose background repeated image is the first image (or filled stars). The width of he inner DIV depends on the rating. User interaction would be similar to sliders, with mouse drag replaced with hover.