The one I like is made by MPI, called Super Glow. The basic one sells for around $40; has three modes:
1-manual, you turn it on and off whenever you want with a Tx switch
2-automatic, turns on whenever throttle drops below a preset point (you set the point)
3- automatic with manual override, you can turn it back on at throttle settings higher than its automatic turn off point. It has a red LED that is on whenever there is current to the plug, and won't come on if the plug is bad, so it's a good check of plug condition.

You can order this direct from MPI at www.maxxprod.com

It's still like rcpilet says, it's still on at low throttle settings. I also use a three position switch like volfy, but I have mine set up differently.
Upper switch position---glow ON manual
middle switch position, glow AUTOMATIC, throttle cut RUN
bottom position, throttle cut ON (I also have the 9C Futaba)

This way, all my secondary throttle controls are on one switch. When I pull the stick back to idle, and flip the throttle cut, engine quits, then I advance throttle past the glow cutoff point until I shut off the glow battery power (I have the glow battery set up with its own power switch, as is recommended). My floatplane is set up this way, but my L-4 is just set up on manual, using the same switch method as above. Doing this, you can cut the glow on when you want, and turn it off when you want, regardless of throttle setting. On my L-4 I only use it for starting.

I use two "AA" size 1500 mAh NiMH batteries in parallel (total of 3000mAh at 1.2VDC) and I can fly with the glow on all the time and still do three or four 15 minute flights without recharging. The 3300 battery should do well, but might not last as long as the NiMH between charges