JB: this is not a high priority for me to write up myself. So I include links to some perfectly good material that others have written. If you find more resources, feel free to suggest.
Section 10 Writing your own functions from the official document, An Introduction to R
User-written functions on the Quick-R website. Very minimal. The slightly expanded version -- 2 whole pages! -- in the R in Action Book version is much nicer, but oh well.
The block on functions, prepared by Davor Cubranic, from the September 2013 R Workshop run by SCARL for the BRITE training program
Nice R code guides (for better or worse, kind of wordy, like my stuff)
Their "total beginner" guide to Writing functions
Slightly more advanced: Using functions to simplify your scripts
Functions from Advanced R Programming by Hadley Wickham