If you have received an offer and have yet to accept, pending the status of another company from whom you are hoping to receive an offer, it is appropriate to share this fact with the company from whom you are hoping to receive an offer — if handled tactfully.
Normally, a second follow-up may be handled by letter or e-mail, but is best by telephone in this particular instance. (Refer to the prior question: What if, after follow-up, I have still yet to hear back from an employer? for information on second follow up in other situations.)
Reaffirm your interest in the position and ask how the hiring process is proceeding. Indicate you have been extended another offer, yet would prefer this position if it is offered to you, and that you would appreciate any information on the position's status to help you make an appropriate decision.
If handled tactfully this way, many employers may be willing to share both the status of the position and the likelihood of your being extended an offer to help you in your decision. (Do not use this as a ploy to facilitate a decision when you have no other offers — it may backfire on you if the employer is not ready to make a decision and feels pressed; you may be told to go ahead and take the other offer, thus eliminating your candidacy for this position.)