a few raymarching fx that I've found

AO.frag

uniform float aoIntensity;          // {"label":"AO intensity",     "min":0, "max":1, "step":0.01, "default":0.15,  "group":"Shading", "group_label":"Ambient occlusion"}
uniform float aoSpread;             // {"label":"AO spread",    "min":0, "max":20, "step":0.01, "default":9,  "group":"Shading"}

// Ambient occlusion approximation.
// Based upon boxplorer's implementation which is derived from:
// http://www.iquilezles.org/www/material/nvscene2008/rwwtt.pdf
float ambientOcclusion(vec3 p, vec3 n, float eps)
{
    float o = 1.0;                  // Start at full output colour intensity
    eps *= aoSpread;                // Spread diffuses the effect
    float k = aoIntensity / eps;    // Set intensity factor
    float d = 2.0 * eps;            // Start ray a little off the surface
    
    for (int i = 0; i < aoIterations; ++i) {
        o -= (d - dE(p + n * d).x) * k;
        d += eps;
        k *= 0.5;                   // AO contribution drops as we move further from the surface 
    }
    
    return clamp(o, 0.0, 1.0);
}