reFX has released a demo version of Nexus for Windows and Mac OS X (the download contains both). The full version has just been updated to v1.0.9 too.

Demo limitations:

• After 5 minutes volume fades out / in.
• After 15 minutes Nexus quits (you have to restart it).
• You cannot save patches.
• You cannot render audio.
• Settings are not saved with your song.
• Reduced library (a good mixture between file size and quality).
• No skin import / expansion import.