There are a lot of books on this list that I have read and/or own. There's quite a few that I would strongly dispute belong on this list for a variety of reasons, many of them already eloquently espoused by others. At the risk of catching rotting vegetation in the face, here's what I'd like to see added to round out the list to an even 100:
76. The Martian Chronicles Ray Bradbury
77. The Lensmen series E.E. ???Doc??? Smith
78. Tau Zero Poul Anderson
79. Riverworld trilogy Philip Jose Farmer
80. The Deryni series Kathleen Kurtz
81. Venus, Inc. Frederik Pohl & C.M. Kornbluth
82. Gateway Frederik Pohl
83. Berserker Fred Saberhagen
84. A Canticle for Leibowitz Walter M. Miller Jr.
85. The Genesis Machine James P. Hogan
86. Dorsai! Gordon R. Dickson
87. A Case of Conscience James Blish
88. Black Easter James Blish
89. Cities in Flight James Blish
90. Retief at Large Keith Laumer
91. The Sector General series James White
92. The Mouse That Roared Leonard Wibberley
93. Flatland Edwin Abbott
94. Shockwave Rider John Brunner
95. Childhood???s End Arthur C. Clarke
96. The City and the Stars Arthur C. Clarke
97. The Andromeda Strain Michael Crichton
98. The Art of War Sun Tzu
99. Gulliver`s Travels Jonathan Swift
100. The Stainless Steel Rat Harry Harrison

I leave it to others to agree or disagree. However, I think the key issue here is that we all have the ability to be literate, learn about a wide range of viewpoints and concepts, and vigourously debate them without emotional or physical harm inflicted. Isn't that the truest sense of being "geek"?