Following Saturday's release of FullHDMI ($2.99) is teamwin's HDMwIn (free), which was only in alpha just a few short days ago—yeah, Shinzul wasn't kidding when he said to "expect [the app] to change rapidly."
The free app requires root and an "accelerated" kernel to function properly. Before using it for the first time, you currently need to flash (tutorial here) one of two modified kernels—one for Sense and one for CyanogenMod 7—and clear the cache/Dalvik.
Features of HDMwIn include portrait and landscape orientation, auto-rotation, resolution switching, adjustable overscan, and background banners. As of this writing, sound output does not work on CM7 (it works on Sense-based ROMs); teamwin is on it, though, so an update that enables it shouldn't be too far off.
More screenshots, a video of HDMwIn (earlier version) enabling full-screen SNES with Wiimote use on an HDTV, and the QR code below.
Links: Market (EVO) | Market (web)
Thanks, teamwin!