NOTE: Click here for details about some unfamiliar terminology you'll find on these sites (especially re: the Subjunctive!)
www.canoo.net Will conjugate verbs for you AND show you noun inflections (i.e. plural and Genitive forms, etc.). Type a noun or verb into the box at the top of the page, and choose "Dictionary Word Forms" from the pulldown menu just below the search box. Explore the site to see lots more info!
Verbix Verb Conjugator Available for multiple languages.
Cactus2000 German Verb Tables Home of the "KonjugationsHase" (conjugation bunny). Who knew grammar could be so cute? Also includes Passive and "Statal Passive" forms further down the page.
verben.info Here you can practice the verb forms until they become easy. Choose a tense, scroll down and check the boxes to include regular (regelmäßige) and/or irregular (unregelmäßige) verbs, choose how many items you want to see at a time, and click "Los geht's" ("Off we go"). When the questions appear, icons to the right of the verb show you the right answer, or the complete conjugation table for that verb). Available for multiple languages.
grammatiktraining.de Here you can find a selection of grammar practice games. Most require both some grammar knowledge for deciding where you want to go, and some simple video game skills in terms of maneuvering the bug or the car or whatever you're controlling in order to actually get there (e.g. Käfer Karl needs to get to the correct answer through a moving "minefield" of killer spiders). Each game is preceded by a German summary of the grammar being practiced, and a German explanation of how the game works, and is labelled according to the level for which it is intended (A2, B1, B2).