Gravity Forms is a wonderful plugin that lets you create sophisticated forms for your website. There are also plenty of extensions that let you get more creative with it. Take the Gravity Forms Range Slider Plugin for instance: a handy extension with conditional logic that lets you add any number of sliders to your forms. It supports single or double handles.
This extension supports negative and fractional values. It lets you use custom values for slider range. You can enable/disable grids, enable steps, and account for minimum and max values.