Protein Preparation
This is the simplist and the cheapest form of labelling. The protein is produced by expression from bacteria which are grown on minimal medium supplemented with ¹⁵NH₄Cl and wild-type (wt) glucose.

Applications
Using ¹⁵N-labelled protein you can record the standard solution-NMR HSQC experiment. This is a good spectrum with which to check whether your protein is folded and assess the quality of your spectrum to see whether it is worth recording other spectra and possibly proceeding on to other, more expensive labelling schemes. It is also possible to record a variety of dynamics experiments using ¹⁵N-labelled protein as well as N-H residual dipolar couplings (RDCs). If the protein is comparitively small (< ~150 amino acids) you can assign the ¹H and ¹⁵N backbone resonances using ¹⁵N-NOESY and ¹⁵N-TOCSY epxeriments. ¹⁵N-labelled protein can also be useful for titrations with ligands or other proteins with which it forms a complex.

Reference:
L.P. McIntosh and F.W. Dahlquist (1990) Quart. Rev. Biophys. 23 1-38. (Link to Article)