Sneaky is a small but versatile plugin that simply allows players to only appear to be sneaking at all times, without having to hold down the sneaking key.
Many plugins offer a similar ability, but they may cause glitchy movement or trigger anti-cheating measures as they modify the player state on the server-side. To avoid these problems, Sneaky applies this effect on outgoing packets destined for each player, ensuring that any modification is purely a client-side illusion. This approach also permit us to selectively disable the sneaking effect, allowing (for instance) your operators or superusers to see through it completely.
The mod and its source code can be found on Bukkit Dev or Github