Daniel Tenner has written an interesting post on how to make you application viral.

The core model for viral growth is the following: viral coefficient = (average number of users invited by each active user) x (proportion of invited users that actually join or become active) x (proportion of such users that invite others). Daniel provides a checklist of techniques designed to optimize the viral coefficient. The general techniques are as follows:

• Make inviting people a core part of the process.
• Keep pulling people back in
• Be useful even if there are no other friends using the application.
• Remove artificial invitation limits.