The guy behind the webcomic xkcd has set up a bash.org clone for limericks. Seems to have already gotten a good mix of “old classics” and extremely creative new ones.
Check it out at www.limerickdb.com
One of my favorites so far is:
Rob, an odd fellow, designs
Poems of equal-length lines
And he limericks with flair
As his forethought and care
Ensure a word count of 3 9s
I love them!
My particular favorite is clean and well suited for this board:
There was a young lady of Wight
Whose speed was much faster than light
She set out one day
In an Einsteinian way
And returned on the previous night.
Note: “relative” can be substituted for “Einsteinian”
There once was an insect that died
From feelings that he kept inside.
With no way to win,
He did himself in
By committing insecticide.
The tenant in 344
Won’t pee in the pool anymore.
Sounds silly to some,
But not when it’s from
His balcony on the third floor!
There once was a soldier named Fisk
Who said, when the fighting got brisk,
“I’m sorry to say
That I can not stay –
I’ve got only one *”
And here’s one with a little “chemistry”:
Magnesium, helium, lithium,
Potassium, calcium, sodium,
Neon, argon,
Xenon, krypton,
Beryllium, barium, cesium.
As a beauty I’m not a great star
There are others more lovely by far
But my face, I don’t mind it
For I am behind it
It’s the people in front that I jar.
There once was a man from Nantucket
Who kept all his cash in a bucket
His daughter named Nan
Ran away with a man
And as for the bucket, Nantuckit
Pa followed the pair to Pawtucket
(The man and the girl with the bucket)
And he said to the man,
“You’re welcome to Nan,”
But as for the bucket, Pawtucket
Then the pair followed Pa to Manhasset
Where he still held the cash as an asset
And Nan and the man
tole the money and ran
And as for the bucket, Manhasset
I must be getting dumber as I get older, and I’m almost scared to ask…
What is it that Fisk has only got one of?
It may be your getting blinder, not dumber, depending on your monitor. Fisk only has one asterisk (*).
One asterisk. (One ass to risk.)
It was probably the quotation mark that made it difficult to distinguish. My bad. I should have set it off more.
There once was a soldier named Fisk
Who said, when the fighting got brisk,
"I’m sorry to say
That I can not stay –
I’ve got only one ***** "
Without entering into the asterisk discussion…
A creature of charm is the gerbil
Its diet’s exclusively herbal
It browses all day
On great bunches of hay
And farts with a delicate burble
I seem to recall a good number of sheep-related limericks on this board. Where were those posted again?
There was a young man of Dunoon
Who always ate soup with a fork.
For he said “As I eat
Neither fish, fowl, nor flesh,
I should otherwise finish too quick.”
A sage columnist name of Cecil
Once lectured a crowd by his easel
Some kid asked him why
So few words end with “gry”?
Adams said “'Cause your clan’s interspecial!”
When I asked the genius whose work follows whether I should credit him when I quoted it, he said “Hell, no.” So I won’t. 
.
[ahem]
A needle-dicked freak ran amuck
Tried to bugger a canvasback duck
His needle got stuck
And he cried “whatthefuck
Whatthefuckwhatthefuckwhatthefuck?”
To quote the Simpsons:
“There once was a man named Enos…”
My all-time favorite, found on a bathroom wall at UCSD:
There once was a pirate named Yates
Who danced the fandango on skates
He fell on his cutlass
Which rendered him nutless
And totally useless on dates.
Now that the Rubicon of dirtiness has been crossed:
From a bathroom stall at the University of Virginia;
My back aches, my penis is sore
I simply can’t fuck anymore
I’m covered with sweat
And you haven’t come yet
And my God it’s a quarter to four!
Speaking of Fisk:
There once was a fellow named Fisk
Whose deuling was exceedingly brisk
So fast was his action
The Fitzgerald Contraction
Reduced his rapier to a disk.
There was a young plumber named Lee
Who was plumbing a girl by the sea
Said the girl “Cease your plumbing,
I think someone’s coming!”
Said the plumber, still plumbing, “It’s me!”
There was a young girl from Ryde
Who ate a green apple and died
The apple fermented
Inside the lamented
And made cider inside her insides
There once was a chap from Japan
Whose limericks never would scan
When he was asked why
He’d always reply
“It’s because I like to try and put as many words into the last line as I possibly can.”
There once was a sailor from Brighton
Who said to his girl, “You’re a tight one!”
She said, “'Pon my soul,
You’re in the wrong hole
And there’s plenty of room in the right one!”