Many QFTs admit long stable string like objects. Some examples include, domain walls in 2+1-dimensional field theories, Abrikosov-Nielsen-Olesen strings in 4d Abelian Higgs models and of course confining strings in pure Yang-Mills theories. 

The low-energy effective action of long string-like objects in QFTs include the Nambu-Goto action plus higher-derivative corrections. Furthermore, Polchinski and Strominger have suggested that the Nambu-Goto action should be supplemented by an additional term designed to cancel the conformal anomaly when working in non-critical dimensions. Therefore, one can consistently quantize the string by imposing Virasoro constraints on the physical states. 

It has been shown that the first non-universal corrections to the Nambu-Goto spectrum are of order 1/L^7 where L is the length of the string. However, corrections from boundary terms can appear before 1/L^7 and a classification scheme for the possible boundary operators has been proposed. There has also been recent proposals for how to embed the Nambu-Goto-Polchinski-Strominger action into a Polyakov framework.