Not a Steam code, but I didn't want to start a whole new thread to just give one code away.
Good for the StarCraft II: Heart of the Swarm beta:
Your promotional code: 2G4P6X-YVWK-XX7R29-EVD9-E4M9YB
Promotion details:
1. Log in to your Battle.net account
2. If you do not have an account, create a free account at:
www.Battle.net
3. Click on the "Account" button
4. Select the "Add a Game Key" button
5. Enter your StarCraft II: Heart of the Swarm beta key
6. Select the “Redeem Code” button
7. Click on “Continue” to add the beta to your account
8. Select Windows or Mac as your operating system to download the beta client
9. Select "Save File" when the installation .exe pop-up box appears
10. Install the Beta client using the downloaded .exe file
Enjoy!