FBReader reads many DRM-free formats, including single-file HTML (with images). You can try it out on a Windows or Linux Desktop. For multi-file HTML you either need to convert it to an OEB e-book (add a .opf file) or use Sunrise XP (say) to make a Plucker version. FBReader will also read the MOBI files in the Upload section, and from feedbooks.com (its Kindle EVDO services, e.g. feedbooks.mobi and Kindle guide, also work via WiFi on the 770).
FBReader does not read PDF. There is a (DRM-free) PDF reader for the 770, but I have never used it.
There is no subscription involved, unless you need to pay for access to WiFi.