AddonManager helps you keep track of your addons and provides easy ways to access them.

It has a main window that lists all registered addons and their details, as well as a "Mini-Addon bar", where small addon icons can go.

The more addon developers that use it (and it's really easy to use!), the more addons will show up in the list. Give it time =)

For users

Type /addons to see all your registered addons
Alternatively, click on the AddonManager button in the "Mini Addons" bar

For addon developers

Registering your Addon

Add a call to AddonManager.RegisterAddon / RegisterAddonTable in your VARIABLES_LOADED event
Example1. At minimum, you can do this:

    if AddonManager then
        AddonManager.RegisterAddon("MyAddonName", "My addon makes you awesome")
    end

Example 2. Using a custom icon, a config frame, and a mini-addon button:

if AddonManager then
    local addon = {
        -- Information
        name = "MyAddonName",
        version = "v0.01",
        author = "Me",
        description = "My addon makes you awesome",
        icon = "Interface/Addons/MyAddon/myAddon32.tga",
        category = "Other",
        slashCommand = "/myaddon",

        -- Config-Frames
        configFrame = MyAddonConfigFrame, 
        disableScript = MyAddon_Disable,
        enableScript = MyAddon_Enable,

         -- Mini Button
         mini_icon = "Interface/Addons/MyAddon/myAddon32.tga",
         mini_icon_pushed = "Interface/Addons/MyAddon/myAddon32.tga",
         mini_onClickScript= MyAddon_Action,
         -- miniButton = MyAddonMiniButton, -- (obsolete but still valid - see below)
    }
    AddonManager.RegisterAddonTable(addon)
end

Of course, replace "my addon" with your addon
I recommend not printing out that your addon has loaded if you use AddonManager, since the user will be able to see it in the AddonManager list.
In the first example, your addon would be considered a "Passive" addon - its text would be colored green (like passive skills) and you can't click on it.

AddonManager.RegisterAddon

AddonManager.RegisterAddonTable = function(addon)  -- Since v1.042

AddonManager.RegisterAddon = function(name, description, icon, category, configFrame, slashCommands, miniButton, onClickScript, version, author, disableScript, enableScript)

Registers an addon with AddonManager. Adds the addon to the addons list and potentially also to the mini-addons-frame.

Parameters:

name - Your addon's name. Default is "[No Name]".
description - A brief (one or two sentence) description of your addon. Something that fits in a tooltip.
icon - Path to a 32x32 image icon (e.g., "Interface/Addons/AddonManager/Textures/addonManagerIcon32.tga"). If no icon is specified, will use default "recipe" icon.
category - One of AddonManager.Categories; will be used for filtering the addons list. Default is "Other". One of Development, Economy, Information, Interface, Inventory, Leveling, Map, PvP, Social, or Other.
configFrame - If your addon has a config frame or other frame you want to show when your addon is clicked, use this.
slashCommands - Specify any slash commands you've registered so that the user doesn't have to remember them.
miniButton - If you want to display a button on the "Mini-Addons" bar, specify it here.
onClickScript - If you need to special handling when your addon's button is clicked, you can specify a script for it. If this parameter is specified, configFrame is ignored.
version - what version this addon's in
author - who made this addon
disableScript - A script that can be used to disable your addon. Adds the disable option in the AddonManager list if both disableScript and enableScript are specified
enableScript - A script that can be used to re-enable your addon

The Mini-Addon Bar

the old extra XML Button definition is obsolete!
you may still use it if you require special button handling
Example:

    <Button name="MyAddonMiniButton" hidden="true" inherits="UIPanelButtonTemplate" parent="AddonManagerMiniFrame">
		<Size>
			<AbsDimension y="24" x="24"/>
		</Size>
		<Scripts>
			<OnClick>
				AddonManager.MiniButton_OnClick(this)
			</OnClick>
			<OnEnter>
				AddonManager.MiniButton_OnEnter(this)
			</OnEnter>
			<OnLeave>
				AddonManager.MiniButton_OnLeave(this)
			</OnLeave>
		</Scripts>
	    <NormalTexture file="Interface\AddOns\MyAddon\myAddonIconNormal.tga"/>
	    <PushedTexture file="Interface\AddOns\MyAddon\myAddonIconDown.tga"/>
	</Button>

The only things you should change here are the Button's name and the two textures.
Note that you can't have a MiniButton if your addon is passive (because what should happen when the user clicks on it?). A passive addon is one that has no config frame and no custom onClickScript.

List of sample Registrations

AddonManager on RoM Wiki