At Grymes, we typically make the online flashcards for the younger students, but teach our upper school students (grades 5 - 8) how to make their own. One tip - before making flashcards, see if someone has already created the cards you're looking for.
Here is an article from Tech & Learning that lists 15 sites for creating flashcards. I really like Quizlet because it supports audio, images, and Spanish (characters and audio). Besides providing students with standard flashcards, the site also provide other tools - with "Speller", it reads the word to test your spelling; with "Learn", it shows you one side of the flashcard and you have to type the answer; with "Test", it gives you written, match, and multiple choice questions. The students really enjoy the fun games and the fact you can access the flashcards from a PC, iPhone/iPad, or android device.
I used Quizlet to create 3rd grade vocabulary flashcards (Vocabulary in Action Level D). Here's an example of a set of Spanish flashcards I created as a test.
At our school, we curate online activities and keep them on this website. We make specific activities available to students at the appropriate time. Here's a link to our online math activities pages. If you want to track your students' progress, you can pay for a service like Arcademics. Their games are free (you only pay if you want administrative reporting) and the students love playing them. They can practice on their own or compete with their friends and others from around the world.