What an incredible piece of software! Stepmania is a simulator for DDR, ITG, and the like - and does it very well. It's hugely customizable to where you can make it look and feel exactly like an arcade machine, or have a more minimalist style like Simply Love that tells and shows you exactly what you need to know. Feels great to play it with a pad, even the $20 ones from Amazon, however Etterna plays better if you're just using a keyboard.
Years and years of custom content, thousands of songs and song packs to choose from - you can't go wrong with Stepmania. It does both casual and competitive play incredibly well, and you can even set it up to be in a paid dance machine if that's your thing.
The default skin works well enough, but if it's not your style check out the hundreds of custom themes and noteskins! Pretty much if something isn't how you like it, there's a way to change it.
The best part of all of this is that it's open source and free. Try it out if you're looking for a DDR or ITG replacement at home, or if you're wanting to get into 4 panel games! You won't regret it, there's a reason DDR is such a well-recognized name.