I’ve been a Digger fan since my childhood. I was recently searching for a downloadable version or online version for free found these links.
You can download DOS version of Digger for free from this link: http://www.digger.org/download.html
It’s best if you can download as you can play the game even if you are offline. However, if you prefer playing it online, here’s the link: http://www.digger.org/java.html
Apart from the DOS version, they have Windows / Unix version too for free download.
Tip: To make the ‘Digger’ experience more exciting you can play the game upside down, i.e, by flipping windows screen upside down. I’m trying to train my mind to play the ‘up-side-down’ version and it’s quite a challenge. Must try I suggest 
good game
good game