Google's Android Emulator
Homepage: http://developer.android.com/tools/devices/emulator.html
Advantages: Runs on every Windows or Linux system. Emulating a ARM CPU means behaviour very close to usual mobile phones. Totally free.
Disadvantages: Not user friendly, some tasks require command line. Emulating a ARM CPU makes it slow.
BlueStacks App Player
Homepage: http://www.bluestacks.com/app-player.html
Advantages: Uses GPU, usually making it run faster.
Disadvantages: Requires a "supported" GPU and 2 GB RAM. Does not run on any of my virtual or physical Windows XP installations. Some features may end up in the commercial pro version.
Jar-of-Beans
Homepage: http://code.google.com/p/jar-of-beans/
Advantages: Community driven, totally free.
Disadvantages: Requires Intel HAXM acceleration, only availble on CPUs with VT-x and EMT64.support.