RAC Licensing in standard and enterprise edition

Standard Edition
RAC option free
Maximum two nodes
Maximum four CPUs
Must use Oracle Clusterware
Must use Automatic Storage Management (ASM)
No extended clusters

Enterprise Edition
RAC option 50% extra (per EE license)
Maximum number of Nodes 100
No limit on number of CPUs
Can use any shared storage (ASM, CFS or NFS)
Can use Enterprise Manager Packs (Diagnostics, Tuning..)