I haven't come across any headphone recommendations here yet.
 
Is it possible to get a decent pair of flat response phones for under $100? 

there have been a lot of threads on this subject.  here are a few:
 
http://forum.cakewalk.com/Headphone-recommendations-m3228531.aspx
 http://forum.cakewalk.com/FindPost/2563886
http://forum.cakewalk.com/FindPost/2580132
 
for under $100, my personal recommendation is the ATH-m40x
http://www.musiciansfriend.com/pro-audio/audio-technica-ath-m40x-closed-back-professional-studio-monitor-headphones

tons and tons of threads here.....
 

feast:
 
http://www.headphone.com/
 

One spec that I would suggest you pay attention to is the impedance.
 
In the case of Beagle's excellent recommendation (Audio-Technica ATH-M40x), they are 35 ohms, usable even on a smartphone or tablet. My mixing headphones are 250 ohms. My tablet gets a hernia even trying force a squeak out of them. On the other hand, the headphone amp on my audio interface (and the others I've tried) can drive 250 ohms no problem.
 
So, consider ALL of your uses before picking a pair. If they'll be dedicated to music production and you'll be driving them with your i/f's amp...forget I even brought up the subject. If they'll also be used mobile, without an audio i/f to drive them, you'll want to stay under 70 ohms or so.

...don't know why when I punched in 'headphones,' nothing came up.
Thanks for the responses.

jones082
...don't know why when I punched in 'headphones,' nothing came up...

 
The forum's search engine doesn't work properly. Use Google instead.

+1 on the ATH M40x. I asked for them specifically for my birthday last year. They are flat. Make sure you periodically compare them with your monitors, though. Headphones just don't have the mass to drive the lowest frequencies, at least in the same physical sense that monitors do.
Mix too much with headphones and your likely to over compensate by adding bass you don't really need to.

Just some random thoughts here: 
 
 
 
If the phones are mainly for tracking, you probably want closed back to minimize bleed.  The ATH M50's
do the job well and can sometimes be found on sale.
 
For mixing, I personally prefer open back phones - they sound a little more natural to me.
I like the Beyerdynamics, (They do cost more, but are a great investment and are also nice just to listen and enjoy music.  I also have enjoyed Grado brand phones - which are a lot of bang for the buck. 
 
Best of luck!  Tell us what you decide on.

A few months ago I asked this question, and based on recommendations, I went with the KRK-8400s. I was able to find them for a little more than $100 but I am very happy with them.