MusicTheory.net is an all in one free resource with lessons and exercises to work on various concepts in Music Theory : Click here to access and try it out!
The Rhythm Randomizer is a site that will randomly generate various rhythms and patterns for you to practice on your own. You can customize, the length, difficulty and concepts you wish to work on. Click here to try it out!
These are the Ed Sueta Rhythm Charts. They span rhythms you would learn in elementary school all the way to rhythms so complicated you would never see them in music. If you are looking to better understand rhythm, these are a great method!