Some of these are still the same from my previous post, but there is some new information here.
7/11 convenience stores - 2127393302
McDonalds - 16005252 (the McDelivery phone number)
CJ Internet Hello D & SO070VOIP & Hello Wireless- 534f4b4354
Starbucks - (branch telephone number; check receipt)
VIPS - ID: SKtelecom_tspot & Password: sktelecom
Tobis - 1234
SK - a123456789
SK T-spot - sktelecom
SK_WLAN - 987654321b
myLGnet & myLG070 - 123456789a, 987654321a, 1234567890
U-Zone - a0123456789
LG Uplus - lguplus002 & lguplus100
U+100 - Terminal's serial number
Of course I can't guarantee that these will work. These are default passwords, so it's likely the owner has changed them, and I've noticed some are assigned new passwords upon installation. But as a test, I walked around my neighborhood and found three connections worked with these passwords, so you just might get lucky. Good information to keep in your back pocket on your Korean trip, anyway.