The Melting Pot - http://www.ddr4pc.cjb.net
          The site when it comes to DDR simulation on PC. You will find simulators, songs, other pad projects and help about everything related to DDR simulation on PC. It's a must see site, check it out (you've probably found my site there, so you already know how good it is).

Delight Delight Reduplication - http://nickle.hey.to
          The first DDR simulator I tried. It's good, I still use it for some songs, but its autor is japanese, so it's very hard to contact the autor (and ask for some new feature, for example). The webpage is in japanese too, so you will have some trouble if you don't read japanese (like me).

DWI (Dance With Intensity) - http://dwi.claws-and-paws.com
          This is a new DDR simulator made by SimWolf. The first good thing is that it's the first public DDR simulator that is not japanese, so I feel more confortable using it and especially browsing its website (since I don't read japanese). It's pretty good but it's software based, so you need a good computer to enjoy it.

StepMania - http://www.stepmania.com
          The newest DDR simulator. It became public recently, but according to its website it's in the works for some time. Its best feature is hardware acceleration, so if you have a good 3D card it will have a great speed in your machine.