Adaptoid which takes a N64 controller. Works best for me with emulation and PC games as it can go analog or digital quite well so playing a platformer or a game like Wing Commander works either way. The ONLY drawback is a rare case like the Geometry Wars excellent rip off Grid Wars 5.4 where it won't run at best without dual analogs.

Adaptoid also is nice as it allows use of any N64 addon pack such as rumble (any rumble emu/win game will use it,) store n64 saves off a ram pack or load games via n64 emu works too (and you can go between cart and pc rom as well safely.)