In no particular order… 
1. Having a great dream, and remembering it vividly after waking up.
2. Sharing a smile with a cute stranger.
3. Getting invited to a party.
4. Making new friends and hanging out with old ones.
5. Publishing a great article.
6. Solving a tough math exercise I’ve been stuck on.
7. Making passionate love.
8. Eating the delicious curry cooked by my cute girlfriend.
9. Rediscovering an old song I used to love and somehow forgot about.
10. Getting links to my website from other people.
11. Taking a nice luxurious shower.
12. Working out ’til my muscles can’t take it no more.
13. Singing karaoke (Japanese style).
14. Wearing a good-looking new shirt for the first time.
15. Getting presents for no particular occasion.
16. Drinking a cup of good tea.
17. Finishing a big project and having the sense of a job well done.
18. Seeing cool sites out the window of an airplane.
19. Autumn.
20. Drinking with friends and loved ones.
21. Watching someone beat a video game in record-breaking time.
22. Reading anything which revolutionizes how I think about something.
23. Texting with friends/girlfriend.
24. Playing Katamari Damacy (and its sequels).
25. Watching Urusei Yatsura or any other great anime.
26. Climbing trees.
27. Exploring places I’m not supposed to go 
28. Watching technology/civilization/the world advance over the years.
29. Kissing my girlfriend and getting kissed by her.
30. Finding coins or other money laying around.
31. Getting/giving a good massage.
32. Wearing clothes which are still warm from being in the dryer.
33. Seeing the politicians win that I voted for/supported.
34. Showing public displays of affection.
35. Lucid dreaming.
36. Getting an A on a hard test.
37. Seeing my friends succeed.
38. Snuggling up before bed.
39. Stereographs.
40. Getting to the bus stop right exactly as the bus arrives.
41. Riding the Shinkansen (Bullet Train) in Japan.
42. Getting so caught up in a novel that I can’t put it down.
43. Being blown away by a great movie.
44. Telling a joke and making people laugh.
45. Being present.
46. Driving around with my girlfriend.
47. Seeing a cute girl checking me out.
48. Helping a student understand a math question they were confused about.
49. Giving a good public speech.
50. Looking good in a picture.
51. Getting complimented for my shirt/hair/smile.
52. Discovering a great new band.
53. When a warm front suddenly comes through and ends a week of cold weather.
54. Getting fan mail.
55. Seeing a prediction I made come true (provided it’s a good prediction).
56. 5-Second Films.
57. Seeing my webtraffic grow over time.
58. Dancing at 80s night at the club.
59. Waking up to teach and finding my girlfriend made lunch for me.
60. Going out on a date.
61. Eating fantastic Italian food and sipping fine wine with my girl.
62. Going to watch a great movie at the theater.
63. Five-, six-, or seven-day weekends.
64. Computability theory, ordinal analysis, large cardinals, etc…
65. Going to the beach.
66. Discovering I have something unexpectedly in common with a friend.
67. Pulling off some wicked tight computer programming.
68. Seeing a big new update to an open source project that seemed dead for years.
69. Cool poetry (like Rime of the Ancient Mariner or Kubla Khan).
70. Going somewhere with a clear sky and seeing bajillions of stars.
71. Introducing friends to other friends.
72. Breaking all the rules and getting away with it.
73. Seeing my students kick butt on the final.
74. Writing a really hard kanji and getting it perfect.
75. Finding an elementary proof for a math theorem that nobody was expecting.
76. Seeing my critics go crazy when I publish a particularly revolutionary essay
77. Writing positive affirmations.
78. Getting a great new haircut.
79. Being accepted for who I am.
80. Loving and being loved.
81. The general weirdness of reality
82. Reading about cool scientific breakthroughs.
83. Looking around underwater with goggles.
84. Relaxing in a hot tub.
85. Stowing away in youth hostels.
86. Discovering new things I never knew about myself.
87. Finding cool/weird/funny sites online.
88. Discovering crazy glitches in popular video games.
89. Swinging on a swingset.
90. Drawing pictures together with a partner.
91. Getting huge bursts of traffic from StumbleUpon and other social bookmarking sites.
92. Lounging around the apartment without a care in the world
93. Being healthy.
94. Brightening up someone’s day.
95. Hearing a foreign language and effortlessly understanding.
96. Knowing that I had a good productive day.
97. Thinking about how awesome the future is.
98. Growing my knowledge.
99. Seeing other people being happy.
100. Being alive!
What are 100 things that make you happy??
FURTHER READING
An Invitation to Hedonism
The Law of Beliefs
One Hundred Reality Escapes
10 Metaphors For Love