Several mechanisms can lead to production of particles during primordial inflation. I will review how such a phenomenon occurs and I will discuss how it can lead to the generation of tensor modes with unusual properties that might be detected in the not-so-far future. The gravitational waves produced this way can have a larger amplitude than in the standard scenarios, can violate parity, and their spectrum can display a feature that can be directly detected within the decade by second-generation gravitational interferometers such as advanced LIGO.