The original "roses are red" poem can be traced back to the 19th century. When the post office started in Britian, people started to send Valentine's cards to the person they loved on the 14th of February. The cards had pictures of flowers and birds on the front and words inside like:

Roses are red, my love
violets are blue,
Sugar is sweet, my love
but not as sweet as you.

The modernized version that most people today know is:

Roses are red
violets are blue
sugar is sweet
and so are you

roses are red, violence is also red, sugar is sweet, and 
so is my gnu.

roses are red, violets are blue, beef jerky chased me
with an axe, and i have red spots on my eyelids from
rubbing them on a tree

roses are red, my beret is black, we are alone in a 
godless uncaring universe, reality is bleak. (and my
turtleneck is too tight)

roses are rented, hester punched the snails, my cutlery
smells like my cat's breath, i have a hernia in my 
ear-wax holder

roses are rented, <insert someones name here> is
 demented

roses are gherkins trained in paramilitary school, a guy
playing the snare drums ate my tulips, sunglasses
sing "mary is really hairy", jumping makes me rub that is
why i have hair there

roses are gherkins, ahhh.... aborted attempt

roses are russian, violets are prussian, when <insert
 someones name here> starts blushin, is end of discushin

roses are red, violets are blue, how much frickin' poetry,
does it take to get a scre-- er, never mind

roses are short and stumpy men with inflated hands carrying
elephant guano popsicles, barry manilow keeps on 
decaying, I can see many things twice and they are 
colourful, where is my margarita you greasy piglet from 
the nether regions of the albatross kamikazee division!?

There's apparently a verse to the song Lavender's Blue that has this:

Roses are red, dilly dilly
Violets are blue
'Cause you love me, dilly dilly
I will love you.

---

There's also a not-very-kind version of "Roses are red" that goes something like this:

Roses are red, Violets are black.
Why is your chest as flat as your back?

Haven't heard it since maybe fourth grade, which is about the same maturity level...