Alright, Mellotrons! Now we're talking! GMedia's Mtron is what you want for Mellotron awesomeness; M-Audio started distributing it bundled with 2 Gig's worth of tape banks. It's about $99 at Guitar Center and places like that; I've had it for a few months now, and although I have an entire hard drive dedicated to realistic orchestral libraries, I still reach for M-Tron's "YES Strings" patch all the time. Unfortunately, somebody left a coffee cup on it for too long, but it sounds fine nonetheless:
For hammond organ, I still feel Native Instruments' B4 is king, but there's also big sample libraries such as USB's Charlie. I don't have it, but I imagine some people may prefer real hammond samples over an emulation. However, if hard drive space is an issue, B4 kicks ass, and it sounds pretty convincing to me.