If you do not remember your username, you can visit www.minecraft.net/retrievename to request an email with your username or a list of usernames registered to that address (multiple usernames were previously able to be linked to a single account; this is no longer possible when creating new usernames).
If you own Minecraft on a Mojang account, you can also log into account.mojang.com and find your username listed under the column "Profile Name".
