I am reading Make: AVR Programming: Learning to Write Software for Hardware and I found this excellent video: Programming AVR Microcontrollers in C – O’Reilly Webcast available from here: Beyond the Arduino: Programming AVR Microcontrollers in C.

The author’s github page is still operational: https://github.com/hexagon5un/AVR-Programming

Their website littlehacks.org is offline but still available via archive.org: https://web.archive.org/web/20160307235812/http://littlehacks.org/

Also there’s this really cool collection of projects over here: Cornell University ECE 4760 Designing with Microcontrollers Final Projects.