Multi-purpose, all-wonderful Sugru can hack just about anything. Add to that list your toothbrush, by easily mounting it to the inside of your cabinet.
One little package of Sugru can make a custom holder for two toothbrushes, as shown above. So as long as you usually buy the same brand of toothbrush (or similarly shaped ones), you can get rid of that toothbrush cup and declutter your sink.
Sugru toothbrush holder [Instructables]