Just about everyone I know has broken a phone at least once in their life. It's almost inevitable considering we carry them everywhere we go, but that doesn't make it any less irritating. A cracked screen is difficult to read and can actually be a little dangerous if there are loose pieces of glass.
Simon, a designer and producer over at Sugru, came up with a quick fix that'll help hold your screen together in the meantime until you can get it fixed properly. He figured out that a paper-thin layer of Sugru will keep the cracks from expanding and prevent any shards from dislodging and cutting your hands.
The process couldn't be much easier. Start by cleaning your screen so there's no dust or loose pieces of glass. Use a few small pieces of masking tape to mark the boundary where your display is (make sure the screen is lit to make it easier to see).
Now, take a teeny tiny piece of Sugru and spread it out over the cracks as thin and smooth as possible. Make sure to stick to the border around the screen and not on the actual display—this stuff isn't see-through.
Once you're finished, just leave it to cure overnight. Since it's such a small amount, it probably won't take the usual 24 hours, but leave it alone as long as you can. Better safe than sorry!
When it's cured, peel off the masking tape and you're all set.
This will obviously make most sense if the cracked area is on the border, or on the back of the device (like with a glass iPhone or Nexus 4), where it doesn't affect the daily use of your display. So, in that regard, it may be a permanent solution. If the crack has already rippled into the usable screen portion, this is best served as a temporary patch.
For more details and step-by-step photos, check out Simon's tutorial.
Cracked Nexus 4 image via Chris Chavez/Phandroid