AllAboutCircuits Forum (http://forum.allaboutcircuits.com/) – A popular forum from the website which catalogs free ebooks on electronics.
Embedded Projects Forum (http://forum.embedded-projects.net/) – A discussion and support forum for Emdedded Projects hardware. They design and sell micro-controller programmers for a wide range of chipsets, to replace OEM programmers such as the AVRISP mkII, PIC Kit 2 and AVR32 programmer.
AVRFreaks (http://www.avrfreaks.net) – A very popular site dedicated to AVR micro-controllers (which are manufactured by ATMEL, however this forum is user created). Find all sorts of information here, from compilers, IDE’s, hardware, 3rd party software, general electronics. Some of the forums are monitored by ATMEL development teams, so bugs and feature requests can be posted here.
EDA Forums (http://www.edaboard.com) – Huge forum about anything to do with electronics