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PCSX2
It's delving into the grey area of emulation but I use it with a clear conscience, playing all my old PS2 games in glorious teneightypee-ovision with a (whisper it) 360 controller. You need a reasonably powerful PC to pull it off, however, especially the CPU.
I actually ordered a new hard drive (160GB for under a tenner brand new!) for my actual PS2 last week. IDE drives are getting hard to find and the one which is in my console currently had previously been running in more than one PC and makes more noise than me getting out of bed in the morning. A hard drive (which only works with the older fat PS2s plus a network adapter) and OpenPS2Loader installed means you can install games directly and run them without the disc with much improved load times.