you have to log in with a paid account once. then you can play offline for free...hopefully someone's ncie enough to provide you with one. i would but it's not mine :3
Just go play classic mode online, because you dont need an account. Then go download one of the Minecraft rips that are out there, preferably the self updating one. Open it and then press force update and bam. Minecraft offline for free.
Just use the cracked version I posted not to long ago you just enter your desired Name and you can either play single or mulitplayer for free.
Check one of the threads here in this forum.
They give you cracked versions