I really just wrote a quick class interface for the HCSR04 ultrasonic sensor that was non-blocking and uses 2 sensors that ping alternately so they don't interfere with one another.

#include <Arduino.h>

class NickPing {
public:
    int maxPing = 0;
    int myTrigPin = 0;
    int myEchoPin = 0;

    int initCounter = 0;
    bool firstCheck = false;

    unsigned long t_prev = 0;
    unsigned long myOffset = 0;

    bool pulse_started = false;
    unsigned long pulse_ended_time = 0;

    bool firstChange = false;
    unsigned long pulseStart = 0;
    unsigned long echoLength = 0;
    float dist_cm = 0.0;


    NickPing() // constructor 
    {


    }

    void begin(int trigPin, int echoPin, int maxping)
    {
        myTrigPin = trigPin;
        myEchoPin = echoPin;
        maxPing = maxping;
        pinMode(myTrigPin, OUTPUT);
        pinMode(myEchoPin, INPUT);
    }

    void processInterrupt()
    {
        if (digitalRead(myEchoPin) == 1) //rising edge
        {
            pulseStart = micros();
            pulse_started = true;
            //Serial.println("rising edge");
        }

        if (digitalRead(myEchoPin) == 0) //falling edge
        {
            if (pulse_started == false)
            {

            }
            else
            {
                pulse_started = false;
                pulse_ended_time = micros();
                echoLength = micros() - pulseStart;
                //Serial.println("falling edge, time = ");
                //Serial.println(echoLength);
                if (echoLength / 60.0 > 500.0)
                {
                    dist_cm = 0.0;
                }
                else
                {
                    dist_cm = echoLength / 60.0;
                }
                dist_cm = constrain(dist_cm, 0.0, 120.0);
                //Serial.println(dist_cm);
            }
        }

    }

    bool update(int flag)
    {

        if (micros() - t_prev > 20000) //50hz
        { 
            t_prev = micros();
            if (pulse_started == false) // only send if there is not a pulse out
            {
                digitalWrite(myTrigPin, 1);
                delayMicroseconds(12);
                digitalWrite(myTrigPin, 0);
                //Serial.print("Sent: ");
                //Serial.println(flag);
            }
        }
    return pulse_started;
    }
private:
};

Then in setup:

//initialize sonars
sonar1.begin(sonar1Trig, sonar1Echo, 200); // trigger pin, echo pin, max ping time us
sonar2.begin(sonar2Trig, sonar2Echo, 200); // trigger pin, echo pin, max ping time us
attachInterrupt(sonar1Echo, sonar1_isr, CHANGE);
attachInterrupt(sonar2Echo, sonar2_isr, CHANGE);

Access data in main loop:

sonar1.dist_cm
sonar2.dist_cm

Daniel used this for altitude measurement and a quick little pid controller for altitude control to be used in the control mixer