Informal e-mail to a friend:

"Write back soon!" 

Semi-formal e-mail to an acquaintance:

" I hope to hear from you soon."

More formal (business letter):

"I look forward to receiving a prompt reply."  (This sounds a bit passive aggressive. I might use it if I were writing a letter of complaint).

"Thank you in anticipation of a prompt reply." (This sounds a bit less passive aggressive but is still a good way of hinting that you want them to write back ASAP).

"I am awaiting your reply," if it is somewhat formal and you feel you are owed a reply.

"I am looking forward to your reply" is more relaxed and friendlier.

"I remain pending" is wrong. The word "pending," incidentally, is very formal and would only be used in business or legal situations. A possible example of use might be:

Dear Mr. Brownlawn

Thank you for your payment. However, we also need a signed copy of the contract we sent you. We are holding your payment, pending receipt of the signed contract, and will commence service when we have received it.

Thank you for your prompt attention to this matter.

Sincerely yours

John Fennelseed

Green Thumb Lawn Services, Inc.