I have founded a go club at a high school in Denmark (https://www.facebook.com/groups/1507387226163610/). After a couple of years, all of a sudden some of my students begin to ask for recommendations for programs or internet sites, and I realize that my knowledge in this regard may be somewhat outdated. For example, I am not sure I would recommend KGS for beginners anymore, since there seems to be far less activity there now than before. Also, I used to recommend Britgo.org for an overview of go-related programs and internet sites, but I suspect there may be better sites for this now? As for programs for mobile phones, I feel on more secure ground, at least for I-phone: Surely, Igowin and Goproblems is a must for beginners?

In terms of servers, OGS is a good suggestion. It is modern, fairly popular, and mobile-friendly.

I've found https://www.ghost-go.com quite useful.