The Monkey Bar is a lightweight (only 0.2 ounces!) adapter bar which replaces the Magpul MBUS PRO mounting clamp for the front sight.
Arisaka makes a number of mounts, from the simple offset, to ring, to inline.
The Unity Tactical EXO seems like a cool product for those end users out there who prefer the Surefire X-Series handgun weaponlights on their rifles.
What are the considerations for a weapon mounted light? Mounting, certainly. Activation process, sure. Brightness? Maybe, maybe not.