This one is really good. Actually I saw this company somewhere in Tom's new book, making things talk. It sells accelerometers, touch sensors, temperature, humidity and pressure sensors. And for me the most interesting sensors are light sensor and color sensor. Also this company provides good sources and descriptions for each sensor like codes of datasheets.
I was surprised when I visited this site because it is Korean site. But you can choose English. Sewori mainly makes ultrasonic sensors and image sensors. I think this site is not fancy at all but it is not complicated to search products so you can see the essential information about the products.
This site offers not only electronics but almost everything. But if you search ‘sensors’ in general, you can see various kinds of sensors such as IR, magnetic, vibration, current, optical, , humidity, gas sensors etc.
This company mainly makes temperature sensors.
Gemssensors makes liquid level sensors, many types of level sensors and also temperature sensors.
They offer many types of position sensors such as rotary position sensors, LVDT position sensors and so on.