Some of the major sins or kaba’ir in Islam are as follows:

1. Associating anything with Allah

2. Murder

3. Practicing magic

4. Abandoning salah (ritual Prayer)

5. Not paying zakah

6. Not fasting during the month of Ramadan without excuse

7. Not performing Hajj, while being able to do so

8. Showing disrespect to parents

9. Severing relations with relatives

10. Committing adultery

11. Committing sodomy

12. Dealing in riba (interest)

13. Wrongfully consuming the property of an orphan

14. Lying about Allah or His Messenger

15. Running away from the battlefield

16. Deceiving the people that one rules and being unjust to them

17. Being proud and arrogant

18. Bearing false witness

19. Drinking alcohol

20. Gambling

21. Falsely accusing chaste women (i.e. of committing fornication or adultery)

22. Stealing from the spoils of war

23. Stealing

24. Committing highway robbery

25. Making false oath

26. Committing oppression

27. Earning ill-gotten gain.

28. Consuming what is unlawful.

29. Committing suicide

30. Lying frequently

31. Judging unjustly

32. Giving and accepting a bribe

33. Imitating the opposite sex (in dress, behavior, and the like)

34. Allowing one’s wife, daughters, etc. to display their beauty to men or conduct illicit sexual relations

35. Marrying a divorced woman in order to make her lawful for her ex-husband to remarry her after he irrevocably divorced her

36. Not protecting oneself or one's clothes from being contaminated with urine or excrement.

37. Showing off

38. Learning knowledge of the religion for the sake of this world and withholding that knowledge (i.e., not teaching people)

39. Betraying a trust

40. Recounting favors

41. Denying Allah's Decree

42. Listening to people's private conversations

43. Carrying gossip

44. Cursing others

45. Breaking contracts

46. Believing in fortune-tellers

47. Behaving badly towards one’s husband

48. Making statues

49. Lamenting, wailing, tearing the clothing, and doing other things of this sort when an affliction falls

50. Treating others wrongfully

51. Treating one’s wife, servant, the weak, and animals badly

52. Offending one's neighbor

53. Offending and abusing Muslims

54. Offending people and having an arrogant attitude toward them

55. Trailing one's garment in pride

56. Wearing silk and gold (this is for men only)

57. Running away from one’s master (this is for slaves)

58. Slaughtering an animal that has been dedicated to anyone other than Allah

59. To knowingly ascribe one's paternity to a man other than one's biological father

60. Arguing and disputing violently

61. Withholding excess water

62. Giving short weight or measure

63. Feeling secure from Allah's Plan

64. Offending pious people

65. Not praying in congregation but praying alone without an excuse

66. Persistently missing Friday Prayer without any excuse

67. Usurping the rights of the heir through bequests (to others)

68. Deceiving and plotting evil

69. Spying for the enemy of the Muslims against Muslim's interest

70. Cursing or insulting any of the Companions of the Prophet

Source: Islamonline