There is so much emphasis on Meditation in Hinduism. Can common meditation techniques of today be traced to scriptural sources? I have heard through word of mouth that Transcendental Meditation technique can be traced to Shiva Sooktam. Like that what are all the techniques available in the scriptures?
The main ancient source of information on Hindu meditation techniques is Patanjali's Yoga Sutras, a very (and I mean very) short work which you can read here. Some people might call the Yoga Sutras scripture, but as far as the major scriptures of Hinduism go, I know of two places where meditative techniques are discussed:
1) Chapter 6 of the Bhagavad Gita (brackets in original):
To practice yoga, one should go to a secluded place and should lay kuśa grass on the ground and then cover it with a deerskin and a soft cloth. The seat should be neither too high nor too low and should be situated in a sacred place. The yogī should then sit on it very ﬁrmly and practice yoga to purify the heart by controlling his mind, senses and activities and ﬁxing the mind on one point. One should hold one’s body, neck and head erect in a straight line and stare steadily at the tip of the nose. Thus, with an unagitated, subdued mind, devoid of fear, completely free from sex life, one should meditate upon Me within the heart and make Me the ultimate goal of life. Thus practicing constant control of the body, mind and activities, the mystic transcendentalist, his mind regulated, attains to the kingdom of God [or the abode of Kṛṣṇa] by cessation of material existence. There is no possibility of one’s becoming a yogī, O Arjuna, if one eats too much or eats too little, sleeps too much or does not sleep enough. He who is regulated in his habits of eating, sleeping, recreation and work can mitigate all material pains by practicing the yoga system. When the yogī, by practice of yoga, disciplines his mental activities and becomes situated in transcendence – devoid of all material desires – he is said to be well established in yoga.... In the stage of perfection called trance, or samādhi, one’s mind is completely restrained from material mental activities by practice of yoga. This perfection is characterized by one’s ability to see the Self by the pure mind and to relish and rejoice in the Self. In that joyous state, one is situated in boundless transcendental happiness, realized through transcendental senses. Established thus, one never departs from the truth, and upon gaining this he thinks there is no greater gain. Being situated in such a position, one is never shaken, even in the midst of greatest difﬁculty. This indeed is actual freedom from all miseries arising from material contact. One should engage oneself in the practice of yoga with determination and faith and not be deviated from the path. One should abandon, without exception, all material desires born of mental speculation and thus control all the senses on all sides by the mind. Gradually, step by step, one should become situated in trance by means of intelligence sustained by full conviction, and thus the mind should be ﬁxed on the Self alone and should think of nothing else. From wherever the mind wanders due to its ﬂickering and unsteady nature, one must certainly withdraw it and bring it back under the control of the Self.
2) The Shanti Parva of the Mahabharata:
Betaking himself to the path of abstention, he should seek to extinguish his dependence on both the External and the Internal. Sitting on kusa grass, with kusa in hand, and binding his coronal locks with kusa, he should surround himself with kusa and have kusa for robes. Bowing unto all earthly concerns, he should take leave of them and never think of them. Assuming equability by the aid of his mind, he should fix his mind on the mind itself. Reciting the highly beneficial composition (viz., the Gayatri), he meditates with the aid of his intellect on Brahma alone. Afterwards he leaves off even that, being then absorbed in concentrated contemplation. In consequence of his dependence on the strength of the Gayatri which he recites, this concentrated contemplation will come of itself. By penances he attains to purity of soul, and self-restraint, and cessation of aversion and desire. Freed from attachment and delusion, above the influence of all pairs of opposites (such as heat and cold, joy and sorrow, etc.), he never grieves and never suffers himself to be drawn towards worldly objects. He does not regard himself as the actor nor as the enjoyer or sufferer of the consequences of his acts. He never, through selfishness, fixes his mind on anything. Without being employed in the acquisition of wealth, he abstains also from disregarding or insulting others, but not from work. The work in which he is employed is that of meditation; he is devoted to meditation, and seeks meditation unalterably. By meditation he succeeds in bringing about concentrated contemplation, and then gradually leaves off meditation itself. In that state he enjoys the felicity which attaches to the abandonment of all things. Having thoroughly mastered the principle of desire he casts off his life-breaths and then enters into the Brahmic body. Or, if he does not desire to enter into the Brahmic body, he at once goes upwards into the region of Brahma and has never to undergo rebirth.
Unfortunately not all the common meditation techniques can be traced back to scriptures. It is because meditation simply means concentrating the mind on a certain object so that it gets absorbed only in that object and becomes devoid of all other thoughts:
tatra pratyayaikatānatā dhyānam [PYS - 3.2]
pratayāntaranirmuktaḥ pratayo dhyānamucyate [Agni Pu. - 373.3]
So depending upon what object is meditated upon, what thoughts or mantras are repeated, and how the concentration of mind is achieved, meditation can be of different types. Prominent scriptures generally lay out some of the fundamental guidelines for meditation. The rest are learnt from a teacher as meditation is a physical process. Keshav has already given the example of the Bhagavad Gita. In purana's like Bhagvata and Vishnupuran the meditation technique is to concentrate on the beautiful form of Krishna or Vishnu.
The meditating sage must think of Vishńu (hold the form internally in the mind), as having a pleased and lovely countenance, with eyes like the leaf of the lotus, smooth cheeks, and a broad and brilliant forehead; ears of equal size, the lobes of which are decorated with splendid pendants; a painted neck, and a broad breast, on which shines the Srivatsa mark; a belly falling in graceful folds, with a deep-seated navel; eight long arms, or else four; and firm and well-knit thighs and legs, with well-formed feet and toes. [VP - 6.7.80-84]
Even before them, the Upanishads mentioned the meditation technique of concentrating on OM to attain Brahma:
स्वदेहमरणिं कृत्वा प्रणवं चोत्तरारणिं ।
ध्याननिर्मथनाभ्यासाद्देवं पश्येन्निगूढवत्॥ [Sve. Up. - 1.14]
Making own body the lower wood, AUM is made the upper,
Churning in meditation through practice, the divine is seen hidden under.
Maharsi Patanjali formed the yoga school of thought by reconciling the scriptures, of which meditation was a limb. Later on Hata Yoga, Laya Yoga, etc. were formed by different yogis and munis.
So through experimentation yogis and saints keep creating different types of meditations while the basic under lying principles remain the same.It varies depending upon what is meditated upon, what mantra is chanted and what is thought about. As per their own preference they give it different kinds of name. Hence, not all techniques preached in the modern times can be traced back to the scriptures, but their fundamentals certainly can be.