The vast majority of AR rifles presently produced, do not come with built-in sights. Instead, their upper receiver has a flat picatinny rail on top. The rear sight is tightened on, either with a carry handle or a small rear sight.
Monolithic Iron Sight Systems
JP Enterprises sells a long single block which attaches to a picatinny rail, and provides both the front and rear sights. I'm unaware of any other monolithic iron sight systems on the market.
Front Sights
Front sights are usually integrated with the gas block, however it's becoming more and more common to replace the front sight post with another small picatinny block. This allows forward-mounted lasers, flashlights, or other accessories.
Integral Rear Sights
The original M-16 was shipped with an integrated carry handle, with a rear sight mounted on the rear of the handle. This has the advantage of being more streamlined than its modern counterpart (a detachable carry handle), but it lacks the rail's flexibility.
Removable Rear Sights
Removable rear sights are typically either fairly small posts, or a removable carry handle which takes up most of the rail space on the upper receiver.
People frequently choose "flip-up" sights, which generally utilize a spring to fold up into position when desired, but otherwise remain tucked low and out of sight. However, these sights are more flexible and thus less consistent than a fixed rear sight.
ARMS #40 flip-up rear sight owners, this page will show you what to replace the mounting nut with if it should get lost.
-- SeanNewton - 02 Aug 2007