Just in case you are one of those people who do not like to re-invent the wheel, here is a list of custom cell renderers other people have written:
Progress bar cell renderer (gaim)
Date cell renderer (mrproject) (is this one easy to re-use?)
List/combo cell renderer (mrproject) (is this one easy to re-use?)
Pop-up cell renderer (mrproject) (what does this do?)
Your custom cell renderer here?!