This could be somewhat addicting.
If you are so inclined, one can scratch an inner-cartoonist caption itch by entering a weekly contest at
The New Yorker.
I did not know I had an inner-cartoonist caption itch until I was presented with an "empty" a cartoon.
The process is very easy: go to www.contest.newyorker.com and create an account. Every week a new cartoon, sans the caption, is there to inspire your wit. Once you have an account you can vote on previous entry finalists, then check back to see who won.
Good luck! And you better believe if one of my captions are ever chosen, I will have business cards printed that just say "contributing writer, The New Yorker."