Minecraft Login BypassONLY WORKS ON WINDOWS.. NOT SURE OTHER THAN THAT. =P
You need to download the latest Minecraft.exe on the official website.. Otherwise this won't work.
Don't worry, this isn't bugged in any way. I wouldn't know how to do that anyway. The only thing I would suggest is to not run it if you're screensharing the monitor. It displays the password you set in the command prompt.
When you first download it, you need to set it up a little bit. Due to my inexperience, you need to put it in the same location as your Minecraft.exe file. Although, you can make a shortcut of the Login Bypass. Oh, and the Minecraft.exe has to actually be an .exe named Minecraft.. =P
When it's all there and good, you need to right click the .bat file and select 'Edit'. You will see this:

You need to set MCName to your username, and MCPass to your username. serverIP1 is the first IP, and serverName1 is just for convenience for you to see which server it actually is. That's it!
When you start up the .bat, it will display your username, password, and 5 choices. If you press '1', Minecraft will startup normally, bypassing the login screen if you have working credentials set up above. 2-5 are the different "ServerIP"s. The .bat should look like this when opened up normally:

The only difference is that I've modified mine not to show the password, even though I deleted the password for this display. =P
Just press one of those numbers, and Minecraft will start up. I'll repeat, 1 is default login, and 2-5 are different server IPs and Names that you've set up.
YES, YOU STILL NEED VALID CREDENTIALS TO USE THIS. IT STILL SENDS YOUR INFO TO NOTCH TO BE CHECKED OF VALIDITY!