Yes there are taxis outside the airport in Malta. However, the ones waiting at the airport come more expensive than if you had to pre-book a taxi online. You can also book a shuttle bus/car (various companies offer this service). Of course you can also always hire a car or take the bus.
Here's a link to some info on car hire ...
http://www.maltabulb.com/malta_car_hire.html
Below are links to some good Airport Transfers companies ...
http://www.maltatransfer.com/
http://www.shuttledirect.com/en/airport/MLA
http://www.maltaairporttransfer.com/
http://www.a2btransfers.com
http://www.maltalowcosttaxi.com/
This is the link to the white taxi service that is available outside the airport ...
http://www.maltairport.com/page.asp?p=17163&l=1
I hope you find the info helpful :)
Cheers and have a fantastic trip!