Vertical foregrips allow the wielder to have a better hold on the gun. These grips allow for better recoil control, but have the adverse effect of lowering a weapon's draw speed.
A gun sling is an over-the-shoulder attachment that allows for easier storage and accessibility. However, gun slings do not provide any other effect other than improved draw speed and a minutely decreased stability rating.
In BRINK, grenade launchers will replace thrown fragmentation grenades. Grenades launched by this attachment will immediately explode on contact with a surface and can be presumably aimed for an indefinite time without risk of accidental suicide.
Bayonets will be included in the "Agents of Change" DLC, along with weapon shields.