In terms of applications, a popular allyl complex is allyl palladium chloride.  
The reactivity of allyl ligands depends on the overall complex, although the influence of the metal center can be roughly summarized as
:(more reactive) Fe ≫ Pd > Mo > W (less reactive)
Such complexes are usually electrophilic (i.e., react with nucleophiles), but nickel allyl complexes are usually nucleophilic (resp. with electrophiles).  In the former case, the addition may occur at unusual locations, and can be useful in organic synthesis.