A powerful note-taking app that includes a "Canvas" feature very similar to Scapple's free-form note-linking.
If Scapple is out of your budget, these tools offer similar "infinite canvas" or mind-mapping features for free: scapple-1-0-keygen
If you want to use Scapple without the risks of pirated software, consider these official options: A powerful note-taking app that includes a "Canvas"