This particular example is called a participial phrase. The whole phrase placed there is an adjective modifying the noun item. Placed is the participle, and there is the adverb modifying the participle. This example isn't actually a complete sentence, unless of course it were the answer to a question. Out of context, it doesn't have much meaning.
Which item did you touch?
The item placed there.
The item placed there is very fragile. Do not touch it.
I'm hesitant to generalize, but I believe any past participle can be used to start a participial phrase. Note that it's not necessary to assume there is an omitted pronoun/subject, that's just the way participles work.
Participle adjectives can go after the noun. Be aware that the placement may change the meaning.
I wrote to the person concerned. (I wrote to the person we're talking about.)
I wrote to the concerned person. (I wrote to the person who is worried.)
There are a handful of adjectives that can go before or after the noun, like stolen or remaining.
The stolen paintings were worth six million dollars.
The paintings stolen were worth six million dollars.
Also when describing size or age adjectives are placed after the noun.
He is six feet tall.
In general, however, adjectives in English go before the noun. Sometimes adjectives may follow the noun at the start of a more descriptive clause, but then they are usually separated with a comma.
The grass, wet with dew, tickled my toes.