Single-sided board developed in KiCAD. Instead of more expensive FT232/AT90USB I used cheap (1.5$) CP2102. Additional connectors for RS232 and I2C (XP2/XP3):
Thats correct, its actually double-sided, but can be manufactured as single-sided without via tenting (or using DIY-photoresist method) and upper side soldered with jumper wires: