Most car rental companies will let you rent a car when you are under 25 years old. However, they almost always charge an extra daily fee that ranges from 15-25 dollars depending on the company. I rented a car from Hertz last week when I needed to drive through FL for a day and had a code that waived the fee as well.
All I did before I made my reservation was google search companies and coupon codes. Since I only had the car for the day it was cheapest for me to just use this code. If you are going for longer you might be better off paying the fee and taking a better car discount.
Sometimes the codes on MouseSavers waive the under-25 fees.
If you belong to a warehouse club (Sam's Club, Costco, BJ's, etc) check to see what their rate codes might get you.
Also check with your employer. Some companies get the Under-25 fee waived and also allow employees to use their corporate rate codes for personal rentals.
When I went to WDW in 2009 my friends and I rented through Enterprise. As others above have mentioned there was an extra insurance for under 25 but we felt it was definitely worth it to have the freedom of our own transportation.
If you plan on traveling other places in the Orlando area I would say that it is worth it.
Now I just stay on property though and don't leave, and deal with the busses.