Google's Android Emulator
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
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.
Advantages: Community driven, totally free.
Disadvantages: Requires Intel HAXM acceleration, only availble on CPUs with VT-x and EMT64.support.
Advantages: Runs on every Windows installation, no special requirements.
Disadvantages: Free only for a 10 day trial period. Long term usage requires commercial.license. Too slow to use on our virtual Windows installations.