Here are a few Advent calenders designed by graphic artists I've found. None are from a Christian perspective, but have 25 days of great graphic designs.
The first is by Rod Barrett
The next is from 383 project.
The last calender is not an online calender, but rather a picture of a real calender. I find this one to be a great design: clean lines, different but complimentary fonts, and a nice warm color scheme. Original found here.