I've noticed that many people are finding this blog looking for playgroups in Seoul. Here is a short list of what I've found so far:
For people with US military base access:
Yongsan Playgroup Forum. This group has a message board, resource center and does playgroups and events. It also lists a lot of the other kid-related events on base. ACS runs playgroups on Yongsan Garrison and Hannam Village. There are also two storytimes: one at the library and one at Moyer.
* SIWA: I haven't been, yet, but I have heard that SIWA has a great and active international playgroup. This group is open to everyone. It is also a good place to meet people involved in non-English, non-Korean playgroups like German, Spanish, French, Swedish, Chinese, Japanese, etc.
*meetup.com has some newcomer/expat groups where you can meet people. You can also start your own meetup playgroup.
* expat groups like AWC, churches, English teacher's groups are good ways to meet people with kids
If anyone else has a good playgroup in Seoul, please leave a comment.