The first thing you should be doing is familiarizing yourself with aspect ratios.
You might be under the impression that a "widescreen" picture has only one type of aspect ratio, namely 16x9, when there are actually more than ten different widescreen aspect ratios, the most common of which are: 1.78, 1.85 and 2.35.
Since your TV is 16x9 or 1.78:1 (16 divided by 9), any video source with a picture wider than 1.78 should give you a letterboxed image. The wider the aspect ratio, the taller the black bars.
However, 1.85:1 aspect ratios will require further explanation:
Almost all widescreen TVs overscan a 1.85 image to completely fill a 16x9 screen. Thus, as a result of the overscanning feature, you should have letterboxing only on pictures wider than 1.85. But the exception is for HDMI inputs, where overscan is usually (but not always) eliminated by manufacturers. Without overscanning, you will still see very slim black bars (sometimes only one very slim black bar) for 1.85 content on most widescreen TVs via HDMI connection.
There are many, many movies with a 2.35:1 aspect ratio, so it would be easy to find examples.
For aspect ratios wider than 2.35:1, the most common is 2.40:1
(actually 2.39:1, but commonly rounded off to 2.40:1). Examples -- Minority Report, The Island, Terminator 3.
The widest aspect ratio I've ever seen on video is 2.76:1
on Ben Hur (Charlton Heston, 1959), shot on MGM Camera 65 (a.k.a Ultra Panavision 70), using 65mm film with 1.25x anamorphic.
2.76:1 image from Ben Hur
The widest aspect ratio I've ever heard of is 4:1
, shot on Polyvision using three 35 mm films projected side-by-side. It was only used once -- on the ending of Napoléon (French silent movie, 1927).