That and effects pedals perhaps as previously mentioned. What about a hardware mixer for hooking everything into or a big midi rig (4 ins/outs minimum from your rig setup)? I'm (when I decide to) using a Korg Extreme 61, Roland V-Synth for hardware synths, NI Komplete 2/4/5 instruments (when I feel like it...), samples, midiman 2x2 (only works currently with my XP install...no Vista 64 support still....). If I was to consider my guitar equipment I have the Line 6 Floor Pod Plus that I can hook in via midi as well which may allow me to muck around more with Guitar Rig 2/3 perhaps. At some point I need to sit down with one of my electrics and mess around and see what kind of madness I can concoct!
Mike