Last year, Mischief Inc. made a minor splash with the release of a free OSR module, Tomb of Rakoss the Undying. Now they are back with the first in a line of 3 modules. The first, The Inheritance, is for levels 1-3.
If it's anything like their first module, it should be more or less compatible with most OSR games, but featuring things like Ascending AC and some other 3e-isms.