I used this app for the first time today at a HPDE at California Speedway. Costs $5.99.

Its a data aq app, basically maps your location using GPS and G forces using the accelerometer, and from that data gives you lap times, split times, and your speed and other data at points along the track. It also outputs a data file that you can use with programs (some of which are free) to overlay guages on your track vids, and make it look like something out of a video game. It can also read your lap times out loud while racing. I wasn't able to get this to play audio through the bluetooth on my helmet... maybe a limitation of my helmet unit.

I used it on Samsung Galaxy s2, using just the phone, no external hardware.

The lap times were quite accurate. On a later session I had it in my pocket instead of jammed snug in the seat cushion, and it was off by up to a half second, probably from me moving around.

Here it is vs my AMB transponder:

AMB________trackmaster__diff
2:30.690____2:30.672____-.018
2:22.353____2:22.352____-.001
2:14.066____2:14.088____+.022
2:08.249____2:08.192____-.057
2:06.260____2:06.275____+.015
2:06.143____2:06.130____-.013
2:17.630____2:17.670____+.040
2:08.404____2:08.399____-.005
2:08.247____2:08.220____-.027
2:04.103____2:04.121____+.018
2:05.031____2:05.018____-.013
2:03.004____2:02.987____-.017
2:02.681____2:02.697____+.016
2:02.356____2:02.356____same

This is using the phone's internal GPS which only records 1 point per second (1 Hz). Some guys buy an external bluetooth GPS unit for $60 that records at 5 Hz and 10 Hz for more accurate data. But I think even without this, it's good enough for substituting an AMB. Unless your good enough to be splitting hairs over tenths of a second. Don't judge my times, there was some traffic in there, and maybe an off track excursion or two ;)

Your mileage may vary, I've heard mixed reviews on very tight courses, especially auto-x. If you have tight turns the app won't know where you were in that half second between points and may not track you accurately. Also if you don't have good GPS coverage at your track, accuracy will probably suffer.

You have to set up the markers for the track using the satellite map, or google earth prior to hitting the track. Easy to do though. The program is also very simple to use, start the program when you hit the grid and tuck it away, it will start recording when you cross the finish line for the first time. I think it also has the ability to use the phone's video recorder and sync it up, but I used a separate video camera for that.

It's useful for studying your laps, for example, I can see how much speed I was carrying through the banked oval each lap, or where my braking points are, etc. Or piece together my best split times for the day and see what my best potential lap time is for my current driving level.

When I get the video results up I'll post a link.

iPhone?

I remember when the early smart phones came out and they had tools like this for diags and tuning. Pretty cool.