There are many keyboard apps in the Play Store. In addition to tobanrob's suggestion, may I add that SwiftKey is very popular because of its powerful prediction/correction algorithms. A.I. Keyboard lets you alter key heights easily, and Kii was very popular last year (though was a bit slow on the 2010 handset I was using at the time).
Many are free, or have free trial periods, so you can try them out and see what works best for you.