Electromagnetic scanner (with gamma pulse generators) - TNG "Evolution"
Graviton telescope - ENT "Breaking the Ice"
Interphasic scanner - TNG "Phantasms"
Lateral scanner - DS9 "For the Uniform"
Long range tachyon scanner - DS9 "Once More Unto the Breach"
Magneton scanner - VOY "Time and Again", "Cathexis", "Learning curve", "Coda"
Targeting scanner- ENT "Silent Enemy", "Storm Front Pt II"
Thermal Guidance Sensor - VOY "Retrospect"
Thermal Interferometry scanner - TNG "Imaginary Friend"
Trans-spectral imager - VOY "One Small Step"
Torque sensor - Star Trek Inssurection, real life
In TNG "Silicon Avatar" sensors of Enterprise D detect disturbance on Melana IV while ship was at warp (but below warp 8). They are "27 hours away at current speed." Assuming warp 1, it gives us range of 27 light hours as low end.
TNG episode "The Chase" states that Federation long-range sensors cannot detect limited-scale life on planet; however, they can detect state of oceans on planet.
Sensors can determine state and capabilities of systems on other ships, but often not if shields are up. Also, high-powered subspace fields were used to block sensors from detecting contents of cargo hold (TNG "The Wounded"); given that ships have subspace field constantly projected by warp drive, plus active shields having subspace field as standard component, it means that subspace fields might limit engagement range in big fleet battles. Sensors can be used to determine composition of most objects, artificial or otherwise, and possible structural damge / instabilities.
Sensors can be engaged in passive and active scan.
In TNG episode "Relics" we are informed that intense gravitational distortions can prevent ships from detecting some objects; however, ships obvoiusly can detect such distortions and drop out of warp to prevent slamming into whatever is generating such distortion.
