• ManyBooks.net - there are a lot of free e-book sites out there, but this one has one of the better interfaces I've come across
• BatteryCare - easy way to get information about your laptop's battery.  It even works with Windows 7.

• Posterous - pretty interesting site that lets you blog by e-mail.  You don't even need to sign up, and you can send pictures, music files, videos, and even DOC and PDF documents.

General

• Anybodyoutthere.com - came across this really unique site through MakeUseOf.com yesterday.  It's got an AJAX-ed interface that lets you chat with strangers without downloading anything or even registering.  I tried it out by chatting with a couple of people and it worked well.  It even lets you chat with multiple people simultaneously from the same browser window, and lets you register and create a profile while chatting.  It's AJAX in full form.

Books

I chose this book because it was well-rated on Amazon.  Overall it's a great introduction to Flex.  I found a lot of typos and code errors in it, but they were pretty easy to spot.  I might have had an early copy since it was from the library, and they might have corrected it since then.