Emulators

FPCE
"Free PC Engine Emulator". An old, open source emulator for DOS (PC-AT/PC-98) and PlayStation. Its source code may still prove useful to curious developers. Maintained by Bero.
http://www.geocities.co.jp/Playtown/2004/fpce.htm

GPengine
A full speed PCengine emulator for GamePark's GP32 handheld.
http://www.gp32x.com/gpengine/

Ootake
A freeware emulator for Windows.
http://www.ouma.jp/ootake/

PC2E
An open source emulator for Windows/MinGW.
http://www.geocities.co.jp/SiliconValley-Bay/9975/PC2E/english.html

RiscPCE
An old, open source emulator for RiscOS. Maintained by Paul Clifford.
http://www.plasma.demon.co.uk/pcengine/pce.html

TGEmu
An open source emulator for BeOS, DOS, MacOS and Windows. Maintained by Charles McDonald.
http://cgfm2.emuviews.com/

XPCE
"DirectX PC Engine Emulator". An open source, FPCE-based emulator for Windows (PC-AT/PC-98). Maintained by Hmmx.
http://www.geocities.co.jp/SiliconValley/7052/