Do you get annoyed that your Minecraft takes too long to start up when you just want to switch to your alt account? Well this mod is for you! There are mods out there for Forge but I didn't see any updated LiteLoader ones, so I took Its_its's outdated AltManager and pretty much revamped it.
How to Use
From the main menu, multiplayer menu, or ingame you can use the key binding to open up the account manager. The key binding to open up the account manager can be changed in Options > Controls; it's set to P by default. Obviously, the list will be blank when you first use it. At this point, I would recommend choosing your secret key first, so click on the [?] next to the text box to see information about what it is, also quoted below:
Your key is the "password" that is used to encrypt your login credentials. Think of it as an extra layer of security so that someone who finds the files on your computer won't be able to immediately steal your accounts. When you add a new account to the user list, the current key you have entered will be used as the password for that account. Logging into any accounts afterwards will require their respective keys.
It is recommended that you use a custom key, since the login credentials are stored in your liteconfig/common/ folder (the password is encrypted using the key), it's remotely possible that a virus steals your information. The text box will save your key for as long as you have Minecraft still open; if you re-launch Minecraft, you will need to enter your key again to switch to different accounts.
Just like the multiplayer server list menu, you can move entries up and down and move them all the way up or all the way down if you hold down shift while clicking on the arrows. Double-click on the entry, click on the login buttons, or click on the right-pointing triangle to use the account.
You'll need to reconnect in order to have the name change take effect if you change while ingame.
If you want to be sure that I'm not stealing your account information, the source code is available here, or you can decompile the mod, whatever, I don't care. For more detailed information on usage, installation, changelogs and such, see the above link as well.