Look for RFID tags, and RFID sensors.

This RFID reader from SparkFun has a built-in antenna and a range of 200 mm and works with inexpensive 125 KHz RFID tags. Also, SparkFun has this USB RFID adapter to connect to a reader such as above, and provide USB data read off the tags directly to your computer or other device.

If you need to span a large area such as a coffee table, your options are: RFID reader with external antenna port, or a number of integrated antenna RFID readers arranged around the periphery of the surface

The mechanism used in some department stores, for instance, has their items tagged with RFID tags, and RFID antenna loops 3-5 feet tall and about a foot wide, on either side of the exit doors. The range is a few feet, which should serve the purpose. In your case, such an antenna would be a suitable loop of wire around the periphery of the surface.

Note that this mechanism will not allow your system to distinguish between tagged objects actually dropped on the surface as opposed to held above it. If the "dropped on" parameter is crucial, then multiple smaller-range RFID readers would serve you better - or a metal contact surface that works as your antenna, designed for a very small read range.

What technique/sensor can I use to recognized tagged objects thrown onto a surface?
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