Sony Playstation

The excellent Bleem! emulator greatly improves the graphics for the games that it supports. The drawback is that it costs 30$, unless you install a crack. There are also freeware emulators like ePSXe.

Bleem Homepage
ePSXeHomepage

Nintendo GameBoy Advance

The first system to be emulated before its release. The discontinued GBAEmu was able to run several demos a few months before the system's release. Newer emulators run commercial games.

Boycott Advance Homepage
DreamGBA Homepage
VisualBoy Advance Homepage

Nintendo GameBoy / GameBoy Color

HelloGB Homepage
Boycott Homepage

NEC TurboGraFX 16 / PC Engine

Magic Engine, the best emulator for the PCE costs $35, so you could use the second best Hu6280.

Hu6280 Homepage
Magic Engine Homepage