http://lobste.rs
I really wish HN would have tags, that's the killer feature of lobster for me (Someone posted http://newshack.io/ a while ago, which is an interface for HN with automatically generated tags, but I don't find it as good).
I've mixed feelings instead toward it being invite only - From one side, the fact that your account is something "precious" can encourage smart/intelligent discussions, and it's also hard for trolls/spammers to get in (and easier for the moderators to ban them permanently).
On the other side instead I like open communities, where the barrier to entry is very low (for example HN, or the always hated 4chan). Not that I can have a strong opinion about the lobster community, as I read it from time to time, but don't have an account on it.