I've got an Audiobox USB from PreSonus to grab the guitar, effectively a nice external soundcard with pretty good analog to digital conversion on it. It takes instruments or mics and can work with phantom power too (not that I need it to.)
I used to have an EMU internal card that did the same thing, but it wouldn't fit in my new computer which had a more compact motherboard. There are advantages to the external box anyway to be honest, so it was a worthwhile investment!
Never tried em live, not played live in a long time, but I'd be happy to try it in a pub or club setting, running the output into a desk and then on to a PA. Perhaps not a patch on the real amp, but not a patch on its cost either
For me it's to let me write and record my own stuff, where I'll record the clean signal in Cubase and indeed apply the Amplitube stuff to it post recording so I keep my options open. Amplitube does work as a standalone too, so I use that for practice, and could conceivably work live.
Anyway, out of the stuff I demo'ed, I liked Amplitube the best. Plenty of other options out there though that people enjoy! In fact it seems people tend to buy one or more of them, which is quite a financial investment if you ask me!