Double-click certain frames^{(see below)} for context-sensitive actions. Generally double left-click to advance and double right-click to go back.
As an example, take or hand in completed quests just by continuously double-clicking the questgiver frame.

Nothing more, nothing less. No extra libraries, in-game options or other fluff nor bloat, just couple hundred lines of double-clicking goodness.

Frames handled:

Quest and gossip frames

Double left-click anywhere to select first available quest^(!), (completed) active quest^(?), or gossip option.
- (Disabled for quest reward item buttons, so you won't accidentally select and accept the wrong item)
Double-click quest reward to select and accept.
Double right-click anywhere to go back or close the frame.

Quest-log and quest-detail frames

Double left-click anywhere to share quest.
Double right-click anywhere to abandon quest. Double-click again to accept popup.

Loot frame and Need/Greed/Disenchant -buttons

Double-click the buttons to select and accept popup.

Tradeskill frame

Double-click any of the list items to quickly craft one selected item.

Auctionhouse frame, Browse and Bid tabs

Double left-click any of the list items to bid. Double-click again to accept popup.
Double right-click any of the list items to buyout. Double-click again to accept popup.

Auctionhouse frame, Auctions tab

Double right-click any of the list items to cancel auction. Double-click again to accept popup.

Vehicle-interface, seat indicators

Double click the yellow dots on the vehicle-interface that appears under the minimap to kick the player/NPC out.

DoubleClickForTheWin