Boogiepop Phantom and Key: The Metal idol are probably the most similar in feel.
Ghost Hound this season is the same writer and director as Lain, but has way different vibes. Still a fantastic show.
People on /a/ complain about Lain being boring, but I disagree. It's a slower paced, much more introspective type of show. I rewatch it every year or so, and always manage to get quite a bit out of it, so it can be deeply rewarding if you've got the patience for it.
If you like the pacing and cerebral aspects of Lain, you'll probably also really like Mamoru Oshii's oeuvre, so give those a look too.