I've recently had a singlespeed commuter built from an old road frame and some parts I had. I did it partly because I was having trouble finding a bike that would fit. She came home on Wednesday and I love her. I was surprised at the number of components when I went through it with my bike shop. I've added the parts I had to the list we wrote out and here it is. Hope it helps.
Frame and fork
Bottom bracket
Headset
Wheels (or rims, hubs, rim tape and spokes if you want to build your own)
Tyres and tubes
Crankset
chain
Brakes and brake levers
Bars + grips or bar tape
Stem
pedals
spacers
cables
saddle
seatpost
Fixed sprocket
Lockring
Freewheel
(All three if you want the option of fixed and freewheel, or you can just fit a freewheel if you're happy with a singlespeed)