my first was a basic calculator
I started with C when I joined cboard, and I ended up with a lite text file and man page viewer for linux in gtk+:
http://www.intergate.com/~halfcountplus/see
which I'm still developing, altho the webpage hasn't been updated in a while.
The only thing I really knew before that was perl, and they (C and perl) shed a lot of light on each other (which makes sense, I believe perl is written in C).
C programming resources:
GNU C Function and Macro Index -- glibc reference manual
The C Book -- nice online learner guide
Current ISO draft standard
CCAN -- new CPAN like open source library repository
3 (different) GNU debugger tutorials: #1 -- #2 -- #3
cpwiki -- our wiki on sourceforge