Yes, the M16 assault rifle is still in active service with the United States Army, typically in the M16A2 and A4 variants. However, it is gradually being phased out in favor of the more compact M4 carbine.
It's a long list, and most of those forces issue it in limited numbers. Aside from the list of users in the Wikipedia article on the M16, an unlicensed Chinese copy is also used by the Iranian Republic Guards Corps, the Sudanese military, and a Chinese made copy of the M4 is used by Peruvian special forces.
As for users of the M16 or licensed variations of it, see the related links section.
Note also that the list is only of "official" users - the M16 rifle has become very widespread, and there are many "unofficial" users. For example, the M16 rifles left over from the US withdrawal from Vietnam and the capitulation of South Vietnam are issued to second line and militia units. Cyprus has the M16 rifle in their inventory, but does not list it in their official inventory due to a US embargo prohibiting US made weapons in the region (although this seemingly only applies to the Greek Cypriots - Turkey stations mostly US-made equipment in the occupied area of Cyprus).
the Americans mostly use M16s or M4s and M9 pistols
Not by the British Army, as they left Austria in 1955. The buildings are still standing and may be in use by the Austrian army.
Yes. The Army tuba players use sousaphones when marching.
Ceremonial use only
The British Army use both the canteen and hydration packs
They remained in service. Some of the original M16s are still in use, albeit with a new barrel, by units of the Air Force. The M16A1 might still be found in some units of the Army Reserve, Army National Guard, and Air Force. In the same of some National Guard units, rather than purchasing complete new M16A2 rifles, they simply bought the A2 uppers and mounted them on A1 lower receivers. Eventually, they were phased out, and either passed on to other countries, placed in storage, or scrapped.
The Army still uses armored cars and light tanks similar to tank destroyers, but not the same kind of tank destroyers as they did in WWII.
The former "Silas B. Hayes Army Hospital" at Fort Ord is still standing and in use. As to what use - I do no know.
Only for ceremonial and reenactment purposes.
A sepoy was formerly the title given to an Indian soldier. In the modern Nepalese Army, Indian Army, Pakistan Army and Bangladesh Army. it is still in use for the rank of private soldier.
All M16s are AR-15s, but not all AR-15s are M16s. Most parts are interchangeable; however, the inside of an M16 lower receiver is milled differently to accommodate the auto sear.
It had been replaced by the Mark 7.