Short of signing up for the developer program and paying $19, what are my options for unlocking my phone to allow loading arbitrary applications? The ChevronWP7 program shut down earlier this month, which was the only other option I was aware of (which was actually just using the developer program behind the scenes.)
If you are a small business that is privately owned, produces software, has less than 1 million in revenue a year and you're not an employee of microsoft, you can apply for Bizspark. Bizspark will get you AppHub access.. which ultimately lets you unlock devices.
edit: I went and looked into my Bizspark account today, and the process of getting a token isnt there anymore. However, if you are accepted into the program you'll be assigned a local Bizspark associate. They might be able to get you a token, but this isnt a guaranteed process. Please "unanswer" this post. Thanks!
If you're a student, you can get a developer unlock for free via DreamSpark, but this limits you to up to three sideloaded apps (vs the 10 from the paid unlock).
WP7RootTools includes a way to set application as trusted meaning they will get full access to your WP7, be careful though as this is an easy way to brick your device if you don't know what you are doing.
Here's a great guide to the various options available for unlocking depending on the device manufacturer. The below information is from the guide:
Samsung 1st, 2nd generation: Interop Unlock possible with WindowBreak, version 7740 and below
LG (all generations): Interop Unlock possible with MFG application
HTC 1st generation: Interop Unlock possible, see Jailbreaking:HTC 1st Generation
HTC 2nd generation: No interop, jailbreak, or custom ROMs possible yet
Nokia: No interop/jailbreak yet