I do HIIT on the elliptical which works incredibly well. I usually warm up for 5-10 minutes, do HIIT for 20-25 minutes, rest for 5 minutes then do steady state for 20-30 minutes (total usually around 60 minutes). This is a pattern recommended in New Rules for Lifting for Women book for people who just can't bear to give up their endurance work. I do different patterns just to mix things up and will do this generally 2x per week (in addition to other cardio and weight lifting). Here are two different patterns that I do on the elliptical that give a good workout:
Patttern 1
10 minute warm up, building up to your base level for HIIT (I will start at level 6 and over 10 minutes work up to level 10).
20-30 minutes HIIT -- 1 minute at level 15, 2 minutes at level 10, repeat as many times as I can handle.
5 minutes off the machine or slow at level 1
20 minutes steady pace, starting at level 7 and going up one level every 5 minutes
Pattern 2 - Ascending HIIT (this one's a killer)
10 minute warm up (same as above)
20-30 minutes HIIT -- 1 minute level 10, 1 minute level 11, 1 minute level 10, 1 minute level 12, 1 minute level 10, 1 minute level 13, 1 minute level 10, 1 minute level 14, 1 minute level 10, 1 minute level 15. At this point, I usually descend through the cycle or start the cycle over.
5 minutes off the machine or slow at level 1
20 minutes steady pace, starting at level 7 and going up one level every 5 minutes
There are also HIIT workouts that include exercise not using machines doing things like burpees, jump rope, jumping jacks, etc. Also, spinning classes will often incorporate HIIT. Fartleks and speed work in running are also HIIT.