C5 provides functionality and data structures not provided by the standard .Net System.Collections.Generic namespace, such as persistent tree data structures, heap based priority queues, hash indexed array lists and linked lists, and events on collection changes
Tooltip plugin: A grumble can be rotated around a given element at any angle, all 360 degrees. Any distance can be specified. Any CSS style can be applied. There's auto-magic size adjustment for use with localised text. FX queues for animating multiple grumbles. And it works in IE6+, and modern browsers.