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NANOGrav (North American Nanohertz Observatory for Gravitational Waves) is a pulsar timing array/PTA-based effort, using the GBT, the VLA, and CHIME (and until 2020, Arecibo). It is an effort to detect gravitational waves through comparison of the timing of observed pulsar cycles. It observes (as of 2023) about 80 millisecond pulsars.
A one-nanohertz cycle (i.e., of nanohertz gravitational waves) has a period of about 30 years, for a the wavelength around 30 light-years or 10 parsecs. A PTA's sensitivity is thought to be highest at about ten nanohertz, and its practical range, roughly 10-9 to 10-6 Hz, corresponding to periods down to around a week and wavelengths down to a few thousand AU. The NANOGrav effort began around 2010 (incorporating some data collected back as far as 2005) and years of data are required to distinguish waves with such a long period (in that, it is something like determining lengthy extra-solar planet orbits from ongoing observations). In 6/2023, the project announced that the NANOGrav 15-year data set shows evidence of a gravitational wave background in the target frequency range, i.e., hints of the Hellings and Downs curve in the collected data.
Similar efforts are the Parkes Pulsar Timing Array (PPTA) effort in Australia, the European Pulsar Timing Array (EPTA), the Indian Pulsar Timing Array (InPTA), the Chinese Pulsar Timing Array (CPTA), and the MeerKAT Pulsar Timing Array (MPTA) in South Africa. The International Pulsar Timing Array (IPTA) is a collaboration of such efforts.
PTAs as gravitational-wave detectors are sensitive to a different range of frequencies than LIGO's and Virgo's 10-10000 Hz regime: the range for PTAs is microhertz and slower (e.g., nanohertz gravitational waves), the presumed detectable sources being binary SMBHs with orbits in this frequency range.