For email, you can use the built-in client or Claws. For podcasts, you can use gPodder. I'm pretty sure there's a password manager on the N900, but I have no idea what it's called. Same for ebook reader (although I'm not sure why anyone would want to read a book on an N900 or GS3 for that matter).

As for Google+ and Twitter clients, I have no clue if there's Maemo clients for them. If there are, then I'm pretty sure they're either old or broken. Perhaps someone else can point you in the direction of those.