Rigidbody.useGravity

Description

Controls whether gravity affects this rigidbody.

If set to false the rigidbody will behave as in outer space.

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour
{
    public Collider coll;

    void Start()
    {
        coll = GetComponent<Collider>();
        coll.isTrigger = true;
    }

    // Disables gravity on all rigidbodies entering this collider.
    void OnTriggerEnter(Collider other)
    {
        if (other.attachedRigidbody)
            other.attachedRigidbody.useGravity = false;
    }
}