Skype has reached its 500th million user download.

The milestone, which is based on global downloads, has been reached in three and a half years starting from the time Skype was first introduced in beta at the end of August 2003.

Skype gives anyone with a broadband internet connection the opportunity to make free or cheap unlimited calls worldwide. However, it faced criticism in January when it announced a new pricing strategy that removed the per-minute charges for SkypeOut calls to domestic landlines and added a connection fee of 0.030 for UK subscribers.

This has not stopped users from downloading the software, with Skype claiming six people download its software application every second. It said free Skype to Skype minutes in the final quarter of 2006 alone totalled 7.6 billion minutes.

Registered users have risen from 54 million in September 2005 to 171 million today.