Even though m-of-n transactions has been supported since the acceptance of BIP 11 in 2011 and BIP 16 the following year, implementations of multisig has been slow going until recently due to lack of support from wallet software.  This will likely change, yet as of today, no address on the Bitcoin Top 500 Rich List uses on-chain multisig (though perhaps some exchanges do in off-chain setups).

BitGo was the first commercial multsig wallet released and Cryptocorp has a neat oracle-based HDM solution to this.

In addition to reading through the relevant threads on StackExchange, be sure to watch James D’Angelo explain how multisig works: