Secular curriculum options are not terribly difficult to find, but they're far outnumbered by Christian-centered options. Reviews and further comments coming as we gather data.

Khan Academy - reading, math, science, history, and other subjects are all covered, and set up to allow students to work independently online.

Time 4 Learning - an online program

ABC Mouse - an online program, appears to be mostly for younger children

Blossom & Root - complete curriculum for K-5, plus some related curricula resources.

Bookshark - the secular version of a more Christian curriculum. Language arts, history, science, and Math-U-See are in their optional virtual program.

Build Your Library - book based, a Charlotte Mason spin off.

Charlotte Mason - while the original is heavily Christian oriented (including daily Bible readings), there are a number of sites that have made secular spin offs (and even a few Pagan folks working on a Pagan one)

Harmony SEL - a free social-emotional learning program.

Our Whole Lives - a comprehensive sexuality curriculum that was written by the UU church, and is nominally secular.

Woke Homeschooling - The curriculum is called Oh Freedom! This is an inclusive history curricula. One set for grades 3-7, one for high school, and an online option. The original version was not secular, but not overtly objectionable; the 2nd edition is secular and provides an optional "religious" (read: Christian) supplement for additional thought and prayer. Predominately African-American, but includes stories from Indigenous and Immigrants of Color.

Back to the Roots - a free gardening curriculum, they also sell tools and seeds to go with it.

Slow Foods School Garden Curricula - 2 curriculum units, plus info on how to run a school garden and how to get that produce into the cafeteria

Duolingo - highly rated language learning app that covers a number of languages

Babbel - highly rated language learning app with a number of languages

Memrise - lessons are of varying quality, but often are created by native speakers. Lessons generally cover listening, reading, and writing.

Rosetta Stone - well known language learning app with numerous languages

Pimsleur - well known language learning method, originally based on tapes in the 80s and 90s.

Middle grades gardening - written by a Black farmer who is active on facebook and offers other resources for schools.