Arduinowww.arduino.cc - The home of the Arduino project.
http://shieldlist.org/ - A list of loads of Arduino shields and pin usage data.
www.jeelabs.org - A variant of the Arduino but smaller and inc. wireless comm's.
http://wiki.hackspace.org.uk/wiki/Project:Nanode - Another Arduino variant inc. an Ethernet controller and HW for a serial link, allowing Nanodes to talk to each other via some 4 wire telephone cable, and they're very reasonably priced.
http://www.practicalmaker.com/ - A good site for some interesting projects, especially if you're interested in aquariums. There's also really good project for a Macro programming language over the serial link to setup Arduino.
Free PC SoftwareThis post lists hundreds of different PC software mostly free but there are some trials and time/function limited ones.