Versus - Find popular alternatives to anything in a jiffy
vsChart.com - With vsChart.com you can compare products and a lot more.
Productz.com - Crowdsourced products database
Diffzi - Diffzi.com lets you compare anything.
Kimovil - Compare price, specs, reviews and benchmarks for smartphones and tablets.
AlternativeTo - AlternativeTo lets you find apps and software for Windows, Mac, Linux, iPhone, iPad, Android, Android Tablets, Web Apps, Online, Windows Tablets and more by recommending alternatives to apps you already know.