Where does the 'nominative object' go?
This is all subsumed under "topicalisation". Chinese does it, but so do others (even Latin!). Indeed, most languages have a way of focusing on something, and Chinese allows certain word orders.

The first example is "object NP fronting" or "object preposing".


  朋友要的词典 玛丽 买到了


comes from...


  玛丽 买到了 朋友要的词典


This is particularly favoured when the wording of the object is long, and especially when it is much longer than the subject. 

The second example is locative inversion.


  前面 开着 一辆车


(In general the sentence would require a 着 here.)

The more canonical sentence


  一辆车 (在)前面 开着 


... doesn't sound as good for this sentence.

Only specific types of verbs are permitted to undergo locative inversion, specifically unaccusative and passivised verbs. There are very few transitive verbs that can do so, most notably 放 fàng.

These non-SVO orders have several different functions. This locative inversion has two main ones: describing the landscape, and presentation focus. These two are relatively similar to English, although the specific number of verbs allowed by English in this construction is lower.