OGS (https://online-go.com/) is pretty good as well. It's web based and supports...

objectobject on March 9, 2015 | parent | context | favorite | on: Number of legal 18x18 Go positions computed. One m...

OGS (https://online-go.com/) is pretty good as well. It's web based and supports both correspondence and real time games. It's also pretty close to KGS in terms of features.