.hack//Sign, Angel Beats! Recommendations
Themes of memory and identity, being trapped in another world, computer programs, the sense of a larger plot not being fully explained... There are plenty of big differences between them, but both shows have these elements,and they're both very enjoyable in their own regard. A lot of thought seems to have gone into their concepts. Plus, the creators of Angel Beats! seem to have ambitious plans, so it may come to have the same kind of multimedia project kind of feel that .hack's been expanding on for so long.
Both series feature some kind of virtual world, Angel Beats takes place in some sort of virtual afterlife and .hack// inside an MMORPG. They both feature a main character who's lost his memories.