When name-changing is released, all non-premium accounts will be released. What this means is, if you have a Minecraft account but have not bought the game, your username will be released into circulation for others to take, due to name changing.
The below link can be used to see if an account exists/is premium. Simply change "USERNAME" to whatever name you want to check. When name-changing releases, you can see if you can grab up some of these. Of course, one username per account, and there is a limit on how many times you do it, yet to be announced. Likely per-month/week/etc. system.
https://minecraft.net/haspaid.jsp?user=USERNAME