Formaldehyde (or closely related compounds) was necessary to properly
stabilize C-41 films until recently. Most modern films, though, are
intended to be processed without the use of a formaldehyde stabilizer.
Kodak now sells a product called "final rinse" or something similar for
use with these films. Using this product with older films, though, may
lead to accelerated fading. Kodak has some information on this on their
Web site, but I'm afraid I don't have an exact URL handy.

Tetenal C-41 Rapid Kit uses a formaldehyde-free stabilizer.  I believe
that the Kodak Flexicolor final rinse is a surfactant only, and does
not contain a stabilizer.  Kodak claims that their new films (Gold,
Portra, MAX, etc.) do not require stabilization.

That being said, I find the stabilizer quantities supplied with most
kits to be inadequate if you don't run a lot of film.  The working
solutions don't store well, IME.  I end up making my own stabilizer
with PhotoFlo and formalin (which I buy from an aquarium supply house
in pint bottles - it's an antiparasitic agent).  Per liter of rinse, I
add 5ml of PhotoFlo and 10ml of standard 37% formalin to 500ml of
distilled water, and then add water to a final volume of 1000ml.  The
formaldehyde concentration in this bath is 0.37%

Disposal of said rinse is regulated by your local authority.  Where I
live, I can legally dispose of up to 100g by weight of formaldehyde
into the public sewer per day.  At the above concentration, I'd have
to dump 27 liters of the stuff to hit that limit (3.7g of formaldehyde
per liter).

When working with this solution and when handling film wet with it, I
would suggest nitrile rubber gloves (available anywhere
surgical/medical supplies are sold - they're typically labelled
"latex-free", but not the PVC kind), which come in cool colors like
bright purple. ;-)

You could also purchase just the Stabilizer from Kodak, a little goes a
long, long way and mix the rest of your formulas from "scratch".
darkroommike