Pcsx2

Pcsx2 is the most complete Playstation 2 emulator to date. This project has been active for a long time, and the developers' efforts become evident in the latest versions.

The emulator is capable of running Playstation 2 games at decent frame rates and it emulates every single feature of the original PS2 console. The games are played with a keyboard by default, but Pcsx2 has good support for other peripherals. If you want to do something really bizarre, you can get one of those Xbox 360 gamepads and use it with Pcsx2. But most gamepads are supported.

The configuration screen is really not something intended for a novice user, but with some help, mostly everything can be customized. There is a console that displays all the information returned by the emulator, which comes in handy for troubleshooting. Out-of-the-box, the commercial games are not supported. To play those games you have to download a copy of the PS2 bios, which can't be distributed with the emulator for legal reasons. There is a compatibility list which shows the supported games and how playable they are. Most mainstream games play really well. Of course, the faster your computer is, the better results you will get.

In short, Pcsx2 cannot be beaten. It is the most complete PS2 emulator out there, and even though it is not the most intuitive one, that is something you will have to live with. It isn't THAT hard after all.