I agree with the others: spin bikes and resistance trainers (or rollers I'd guess, though I've never used them) are great during the winter and can definitely keep your fitness up. In fact, I know someone who, for job reasons, had to switch all his training indoor after 20 years of regular training and actually improved his performance. While I wouldn't recommend that approach, I do think that trainers are particularly well suited for interval workouts--maybe even better than the road where you have to deal with stop signs and such. The only intervals I've found better more effective are those done behind a moped or scooter, and I don't have access to that now or over the winter.