One great option for creating data to populate your sandbox or Dev Org for general testing is to go to a site called GenerateData.com, specify the field names you want, the type of data the field contains, the field's format, number of records, etc., and it will generate random data for you which you can use to import into your org to use for testing.
The site lets you select the format you want the data saved in for download. You can have the data output as either HTML, Excel, XML, CSV or SQL. Up to 200 records can be downloaded at a time for free. If you donate $20 or more, you get an account that lets you download up to 5000 records at a time with your settings saved for up to a year. This is one of the most useful sites I've found if you're looking for to generate random test data of this type.
Obviously, you can use DataLoader or another appropriate utility to import the data into your org once you've downloaded it.
I should add that there's also a GNU licensed version you can download that allows you to create as much data you desire provided you have a web server available to set it up to run from.