What pauper cards are legal in MTGO but not paper?
I have a program that lets me do weird queries, this was an interesting one to code.  Here are the results:

Adarkar Sentinel                  Addle                             Aeolipile                         
Afterlife                         Alluring Scent                    Anaba Ancestor                    
Anaba Spirit Crafter              Armored Griffin                   Arrogant Wurm                     
Balduvian Conjurer                Barktooth Warbeard                Battle Screech                    
Beetleback Chief                  Bone Flute                        Brass Man                         
Brilliant Plan                    Brindle Shoat                     Chainer's Edict                   
Circular Logic                    Citanul Druid                     Claws of Wirewood                 
Coal Golem                        Dauthi Mercenary                  Death Spark                       
Demonic Torment                   Desperate Charge                  Drift of the Dead                 
Ebony Horse                       Elephant Guide                    Elven Lyre                        
Exile                             Fellwar Stone                     Feral Thallid                     
Fire Drake                        Flying Carpet                     Foul Spirit                       
Gilded Light                      Glasses of Urza                   Goblin Commando                   
Goblin Firestarter                Goblin General                    Gravebind                         
Heavy Fog                         Hunting Cheetah                   Hyalopterous Lemure               
Icatian Lieutenant                Icatian Phalanx                   Icequake                          
Icy Prison                        Ironhoof Ox                       Jedit Ojanen                      
Jerrard of the Closed Fist        Just Fate                         Killer Whale                      
Kjeldoran Elite Guard             Kjeldoran Skycaptain              Knights of Thorn                  
Kobold Taskmaster                 Lady Orca                         Lava Flow                         
Lesser Werewolf                   Library of Leng                   Lightning Blow                    
Mana Prism                        Mightstone                        Mistmoon Griffin                  
Oasis                             Obsianus Golem                    Ogre Taskmaster                   
Onulet                            Peach Garden Oath                 Phantasmal Forces                 
Phantasmal Mount                  Phantom Monster                   Predatory Nightstalker            
Primal Clay                       Ramirez DePietro                  Red Cliffs Armada                 
Righteous Avengers                River Merfolk                     Rockslide Ambush                  
Roots                             Royal Trooper                     Sea Sprite                        
Shield Sphere                     Shu Elite Companions              Shu General                       
Shu Soldier-Farmers               Shyft                             Sir Shandlar of Eberyn            
Sivitri Scarzam                   Skirge Familiar                   Skirk Drill Sergeant              
Slashing Tiger                    Sol Grail                         Soltari Emissary                  
Spoils of Victory                 Strategic Planning                Tangle                            
Tawnos's Wand                     Telekinesis                       Thallid Devourer                  
The Lady of the Mountain          Thermokarst                       Thrull Retainer                   
Tobias Andrion                    Torsten Von Ursus                 Tribute to the Wild               
Trip Wire                         Wall of Kelp                      Wall of Light                     
Wanderlust                        Warp Artifact                     Wei Elite Companions              
Whiteout                          Wu Longbowman                     


EDIT: forgot Tempest Remastered:

Armor Sliver                      Barbed Sliver                     Bottle Gnomes                     
Elven Rite                        Horned Sliver                     Mnemonic Sliver                   
Patchwork Gnomes                  Skyshaper                         Staunch Defenders                 
Telethopter                       Verdigris                         Whispers of the Muse