4 cores supported:

Creativity      Adobe   After Effects 7/8      
Creativity      Adobe   Premiere Pro 2/3.0     
Creativity      Adobe   Encore DVD 2.+ 
Creativity      Maxon   Cinema 4D v10 (TBD)    
Creativity      Pinnacle        Studio dv 10   
Video   Sony Media SW   Sony Vegas 7.0 
Music   Steinberg- Yamaha       Cubase v4.5    
Games
THQ/Gas Powered Games   Supreme Commander
Remedy Alan Wake
Valve Half-Life 2: Episode 2
Epic Unreal Engine 3
Ubisoft Splinter Cell: Double Agent

Additional Creativity/DCC 4+core support
Autodesk 3DSMAX 8 and 3DSMAX 9
Adobe Photoshop CS2
Quicktime Pro 7.1
MPEG with DIVX 6.2
DVD Shrink 3.2
Pov Ray 3.7 Beta
TMPGEnc 2.524     
       
A sampling of applications available or shipping soon that are enabled for two processor cores::
Games   10tacle Elevon 
Games   Activision/Lionhead     The Movies     
Creativity      Adobe   After Effects 8
Digital Content Creation        Adobe   Photoshop 10   
Creativity      Adobe   Photoshop Elements 5.0 
Digital Content Creation        Adobe   Acrobat Creator
Creativity      Adobe   Acrobat Reader 
Creativity      Adobe   Premiere Pro 3.0       
Creativity      Adobe   Audition 2.+   
Creativity      Adobe   Encore DVD 2.+ 
Tool    Adobe/Macromedia        Flash player   
Music   Cakewalk        Sonar  
Digital Content Creation        Cineform        Aspect HD      
Games   Codemasters     Race Driver 3  
Games   Koch Media / Sunflowers Anno 1701      
Video   MainConcept/Elecard     H.264 Encoder  
Video   MainConcept/Elecard     H.264 Decoder  
Creativity      Maxon   Cinema 4D v10 (TBD)    
Games   Microsoft/Big Huge Games        Rise of Nations: Rise of Legends       
Games   NCSoft/NetDevil Auto Assault   
Creativity      Pinnacle        Studio dv 10   
Games   Primal Software Heavy Duty     
Video   Sony Media SW   Sony Vegas 7.0 
Music   Steinberg- Yamaha       Cubase v4.5    
Music   Steinberg- Yamaha       Nuendo v4.0    
Games   THQ/Gas Powered Games   Supreme Commander      
Games   Valve Software  Half Life 2; Episodes  
Additional games, 2 core support:
•       Ghost Recon Advance War Fighter
•       EA/Lionhead: Black & White 2
•       Quake 4
•       Activision/Infinity Ward: Call of Duty 2
•       Vivendi Universal: F.E.A.R
•       NCSoft/Cryptic: City of Villains

Intel has compiled a list of applications that are known to support 4 cores and 2 cores. There are other applications, not listed, and I’m sure that more applications will take advantage of 4 cores as more Quad Core CPUs are released, so don’t take it to mean that the list is exhaustive. Of note are the fact that Quake4, COD2, FEAR, GRAW  support 2 processor cores. Many of the upcoming games including HL2 Episode 2 and Unreal 3 support 4 processor cores, meaning improved performance with the Quad Core CPUs.