Nilakantha: Blue throated
When the Devas and Daityas churned the ocean for amrit, first a fiery poison called halahala was produced which threatened to overcome everyone. Shiva Bhagawan saved them by swallowing it. It got no further than his throat which turned blue in the process. Hence Shiva Bhagawan is known as Nilakantha. During the Pralaya when the present cycle of the world is ended, He will vomit forth this poison again to destroy the world. This should not be seen as a negative thing. Just as fire burns but is also called pavaka ("purifier") because it reduces things to their essence, the pralaya is not the end of the story but the beginning of the renewal of the world and the beginning of its new cycle. It is only poison to samsara.
There is an inner meaning too. Shiva Bhagawan is Yogeshwara - the lord of Yogis. It is due to His complete self-mastery that the halahala is contained in suspended animation as it were in His throat. In that state it is nether non-manifest or manifest but inert, perpetually ready to activate its potential.
The halahala is also symbolic of mantras. Whereas to signify memorizing something, an English speaker might say he has it "by heart", in Sanskrit-derived languages we say it is kanthastha "in the throat" Shiva Bhagawan has all the mantras in His throat. In the wrong hands they are poison. In the hands of an expert, they have great power for good.
Some additional notes from Shri V. M. Sundaram
Some more information provided by list member Ashish.
The temple of Shri Nilakanth Mahadev is located about 14 km from Rishikesh. The temple can be reached after crossing 3 hills through a mountain path, or by car. The shivlinga at the temple is considered to be related to the Pauranik katha about the drinking of Halahala by Shivji. The temple was rediscovered in the 19th century by a saint by the name of Shri Siddha Bali Baba. There is a small temple atop the hill from Nilkanth temple that is devoted to Siddha Bali Baba. Both the temples are run and maintained by Brahmacharis/Sannyasis. The Nilkanth Mahadev temple is maintained by a Sadhu of the Niranjani (or Maha Nirvani Akhada - I might have them confused), one of the Dasanami order Akhada, as also one of the seven Shaiva ones. During the month of Shraavan, there is a great Mela that is organized in Rishikesh dedicated to Nilkanth and a great number of devotees visit the temple with Gangajal and Bilva leaves to offer to Shivji. About 45 km from Nilkanth Mahadev are some caves, that are reached by a jungle path only, that are considered to be the abode of several Siddha Sannyaasis that are engaged in Saadhnaa. Needless to say, the entire region is full historical/religious importance and very holy.
Om Nilkanthaya Namah