Matt Cutts gave an extremely interesting and dynamic talk at WordCamp about blogging and Search Engine Optimization. Not only was it extremely entertaining, but it contained lots of interesting tidbits about how to increase the PageRank of your site.
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: