EDIT: nullstring has made a wiki page with this list here: http://www.head-fi.org/wiki/list-of-diy-headphone-amps
I just had a thought: we need a list of current do-it-yourself headphone amplifiers. So here it is. I'll be updating when someone alerts me to new amps or amps that I have missed. Now, since there is a very large amount of circuit designs available, I'll only be listing available kits and PCB (if you want a circuit design only, you should check headwize).
And what do you know? I just found a brand new design that came up yesterday (31/01/2011)!
*****IMPORTANT***** If the link sends you to a thread, read it before posting. Nothing is worse than a newcomer asking questions that have already been answered.
Available as a kit (and sometimes as a PCB).
Available as a PCB.
Available as a group buy (hurry!)
Schematics only. If you're lucky, there might be a bill of materials, or even files to make a PCB.
AMB α20 (info & PCB)
AMB M3 (info & PCB)
AMB Cavalli-Kan Kumisa III (info & PCB)
DAO & DAO with Taylor CCS (info)
Pavel Macura Audio Dispre 2 - JFET (info & PCB)
Sjöström Audio QRV01 through QRV08 (info & PCB)
Twisted Pear Audio Ventus (info & kit)
"The Wire" (info & Group Buy @ DIY Audio)
Starving Student Millett Hybrid (info)
Bottlehead Crack OTL (info & kit)
Bottlehead Single Ended eXperimenter's (S.E.X) (info & kit)
Bottlehead Smack WOT (info & kit)
ecp.cc L'espressivo (info)
Hagerman Technology Castanet (info)
Pete Millett "Jonokuchi" (info & PCB)
Maple Tree Audio Ear+ HD (info & kit)
World Designs HD3S (info & kit)
Decware ZKIT3 (info & kit)
Oatley Electronics K272A (tube hybrid) (info & kit)
PPA (info & PCB)
PIMETA v2 (info & PCB)
Cavalli Audio eXStatA (Solid State & Hybrid) (info)
Kevin Gilmore All-triode Direct Drive (tube) (info)
Kevin Gilmore Blue Hawaii (hybrid) (info)
Kevin Gilmore Current-Domain (KGSS predecessor?) (info)
Kevin Gilmore Solid State High Voltage (thread)
Kevin Gilmore's Stax SRM-T2 clone (thread)
Audiologica (CMoy, PPA & Pimeta)
AMB Laboratories (also stocks PCB for Kevin Gilmore amps)
Beezar (kits for Millett Hybrids)
DIYforums.org (Millett Hybrids)
Electric Sumo (CMoy kit)
Glass Jar Audio (kits and PCB for AMB and Cavalli amps)
GlassWare Audio Design (Aikido)
JDS Lab (CMoy Bass Boost PCB)
ecp.cc (several DIY projects)
Headwize Library (DIY with an enphasis on the Y)