Ah, the treadmill. Modern day torture device.

1. I1: Treadmill Tabata: Bring on the vomit! This is a short and extraordinarily effective workout. Warm up as needed, then set the incline to 10% with your feet on the side-boards. Now get the belt going at your target pace (I recommend no faster than 10 mph--a 6:00 mile--as it is hard, at least for me, to quickly get my legs moving more quickly than this). When you're ready to go, and as the timer ticks over to a whole minute, use strong arms to support your weight and quickly match your running speed to that of the belt below you before letting go with your hands and running up the incline for 20 seconds. Hop to the side-boards using hands on the rails to facilitate the movement. Watch as 10 seconds pass in the blink of an eye. 7 more 20 second 'runs' followed by an equal number of 10 second rests on the side-boards. The first one should feel easy. The last one should feel impossible (and often is). Warm down as needed. This is a bit of an advanced workout, especially when done at near max effort--make sure you are comfortable getting on and off the always spinning belt quickly and safely.
2. I2: The "Hill" Program: Many manufacturers of treadmills have adopted a similar 'hill' program that offers a warm up followed by a short 'tempo' section, then four intervals of increasing difficulty, and finishing with a 2 minute 'warm down'. Choose a base pace for the program as indicated by your Vdot for a 2 mile effort and program the machine for a 10 minute session. 10 minutes is pretty ideal--the work intervals last 30 seconds a piece, with equal length RI. You'll be running the same pace the entire workout--the intervals involve an increased incline. All you'll need to do is choose the pace (9 mph for me) and level (which will determine what inclines are used). Subsequent sessions of the same workout can either increase the pace or the level. NOTE--this workout may not be accessible depending on your machine. Some treadmills don't change the intervals based on the program length, and so a 10 minute session wouldn't get into the guts of it. But both of the following programs should work on any machine.
3. I3: Hill intervals--Similar to the above, but manually controlled. Chose a pace based on your Vdot for a two mile run and start trucking. When you're warmed up, wait until the clock gets to the top of a minute and key up the incline to 3% for one minute. At the end of the minute, key it back down to 0%. Repeat the 'hill' each minute until you've finished 5 intervals, or 10 minutes (plus warm-up) of running. Work on feeling relaxed and bringing your heart rate down during the flat sections, even though you aren't slowing down. I love this workout because it shapes what feels easy mentally. Adding the effort of the hills at an already aggressive pace helps that pace feel almost like a recovery pace when it is flat. Progress on this program can either be via increasing the pace or increasing the incline during the 'hill sections'. I favor the latter for some reason, mainly because it allows for 'micro' adjustments--if the last attempt at this workout felt daunting enough I can just settle for matching it and then, once I'm into it and feeling motivated, still up incline for the last couple of intervals, setting new Baseline. Improvement!
4. I4: Speed intervals--Similar to hill interval, but here the increases in intensity come from running faster rather than uphill. Your base pace for this won't quite be as high as in hill intervals, but then your working interval pace will be considerably faster, somewhere between what you'd be using if you were dong 400 meter sprints outside. For reasons explained below (Pro Tip) I don't really favor speed intervals on a treadmill, unless there is a program that specifically lets you do them in a 'hands off' way. But then again, to each his own!