One of the joys of the Mac community is that if you find something--a program, a method of doing something, what have you--there usually isn't a shortage of people willing to suggest alternatives. That same sort of "Hey, did you consider this?" spirit seems to be emerging in these early years of the iPhone platform as well.
A comprehensive directory with a small expanation of each site beside each logo. It is a very handy overview of what is available in social and business networking.
This directory from the UK also ran a vote for Top 100 in 2007, and is repeating the vote in 2008 - tools that can be used for creating learning and performance support solutions as well as for managing your own personal learning.
UK BECTA site on Educational podcasting for teaching and learning
The definitive dynamic maintained list of Web 2.0