Japanese Verbs - from "Tim in Takamatsu." Whoever this guy is, he's a born teacher. This terrific tutorial (all romaji) goes beyond mere conjugation, and gets to the heart of the spoken language.  For Java flashcards that I rather shamelessly based on Tim's stuff, go here.

All About Verbs - from About.com.

Japanese Verb Conjugation Practice - from Stanford University. You'll need the IME to enter answers.

Verb Conjugator - check it out.

101 Verbs! Java flashcards by Yours Truly.

Sullydog's Challenge Boards - these are Java drills I posted at Quia. Give them a try.

Challenge Boards - Java drills at Quia, all in romaji.

Affirmative!  Drill on the plain and polite affirmative verb tenses.

Can Do!  Works the potential tense.

Didn't Wanna! Here's a drill on the past negative desire form.

Don't Do It!  The negative command form.

Don't Wanna! The negative desire form, present tense.

Go Negative! The plain and polite negative present tenses.

Make Me! Practice the causative tense.

Didn't Do It! Works the plain negative past tense.

-Ta Form! This form is used not just for the plain past tense, but for many other common constructions.

-Te Form! This is an extremely important and versatile verb form.