Yes. Register for a Developer account with Microsoft. Cost is $99/year. And I believe you can unlock up to 3 devices at a time. Oh, and aside from being able to sideload apps (only non-interop unlock apps), you can also develop, as you now have an account.
This is what I have been using the past two years to sideload. Legitimate and legal. But costly. Fortunately, Nokia paid for this years registration with their $100 credit through AT&T.
Sent from my Lumia 900 using Board Express