Depending on the situation at the moment, I have applied "extra" paychecks to car loans, home equity loans, or holiday shopping. I know it doesn't sound very fun, but it gives me peace of mind, and I'd rather do that than pay interest. Paying off loans, and doing so early, makes me very happy.
We used this idea, too! Now that we're debt-free, I buy a CD or savings bond with the 'extra' check. It's an easy way to invest in our future!