Reddit mentions of The Biochar Solution: Carbon Farming and Climate Change

I wrote up half a dozen methods in my climatecolab proposal, skip down to the "feasibility" section. I've got a lot of links.

Two of the biggest are biochar (working charcoal into soil) and farming practices that restore topsoil (no chemical fertilizer, no plowing other than a subsoil plow). Those two alone could absorb a huge amount of carbon, while also improving soil and reducing fertilizer runoff. A really good book on this is The Biochar Solution. Another, which focuses on topsoil restoration alone, is Priority One. That one has some annoying parts (he totally discounts biodiversity), but the stuff on soil is great. He's very optimistic about it.

A super cheap way to get started with biochar is cookstoves.

There's also a technology called "artificial trees," which absorbs and liquifies CO2. One of these can absorb as much CO2 as you'd offset by building 500 similar-sized windmills. At scale they estimate it'd cost as little as $20 per ton of carbon.

Five percent of our CO2 emissions are from cement production; there's a carbon-negative cement to fix that part.

There are a couple other techniques I linked that use the ocean, and this one (pdf) that I found later. Another idea I found recently is this scheme to repopulate tundra with bison and elk, converting it to grassland and stabilizing the carbon there, avoiding gigatons of emissions.

Edit: here's a recent article in Time about how farmers can absorb a lot of carbon.