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Ace(1), Ace2's main purpose is (as stated to WoWace.com) to improve modularity, improve performance, and simplify the addon and library creation process.
Ace2 will replace Ace when it achieves gold status. Although Ace2 is currently under development, many popular addons are already using it's libraries.
Ace2 is Currently at END OF LIFE status, and is being superseded by
Brief feature list and component overview of Ace2
AceLibrary handles version compatibility of libraries - Ace ones as well as your own. It also helps with error handling.
AceOO-2.0 provides an object-orientation framework.
AceAddon-2.0 is Ace's main interface to AddOns. Ace components present their functionality through it. It also handles simple initialization events.
AceComm-2.0 Inter-player addon communications through hidden channels.
AceConsole-2.0 provides console I/O functionality - slash commands, easy printing, etc.
AceTab-2.0 Provides easy tab completion support.
AceDB-2.0 allows for fast, clean, and featureful saved variable access, including per-class settings and more.
AceEvent-2.0 allows for simple event handling, scheduling, and inter-addon communication.
AceDebug-2.0 simplifies AddOn debugging.
AceHook-2.1 provides safe hooking of functions, methods, and scripts.
AceLocale-2.2 makes localization (translation) of your AddOns ridiculously simple.
AceModuleCore-2.0 helps you split your AddOn into separate "modules" that can easily be turned on and off. Handles enabling/disabling of events, hooks, tooltip scanners, etc for you.