Summary
Datamockr is the easiest way to generate mock data on the fly. Simply make a GET request specifying the type of data you want and a sample size.

Example
Here are a few example requests:
GET /data?fieldType=city&size=10

# response
{
    "data": [
        'Toronto',
        'Wellington',
        'Miami',
        'Sydney',
        'Ulicesland',
        'Marcmouth',
        'Koelpinton',
        'London',
        'Edmonton',
        'Sherwood Park'
    ]
}

GET /data?fieldType=city&size=5

# response
{
    "data": [
        'Courtney Leannon',
        'Lee Harber',
        'Patty Ledner',
        'Mrs. Henry Zulauf',
        'Dale Leffler PhD'
    ]
}

For a full API reference, including the fieldTypes you can provide, visit www.datamockr.com.