KVerbos is a free, open-source program that allows you to easily study the conjugation of Spanish verbs.
It works very simply. In each round, it presents you with a verb and a verb tense. Using that information, you have to fill in what it is asking for, which is, of course, the verb conjugated in that tense.
As you advance, you will get a higher or a lower score and the points will appear on the results screen. In addition, you can create users in order to keep track of the results of your different study sessions.
All of this makes KVerbos a complete and educational app for learners of Spanish language.
Remember to install the verbs file in order to best take advantage of the program.