Some are strong, and some are weak. The weak, as is well known, are easily mastered—completely regular and, frankly, pathetic. But it doesn't have to be that way! The Society for the Strengthening of Verbs labors at its noble cause of strengthening verbs and nouns (in English too, though with less Sprachgefühl), increasing the stock of causatives, and generally messing around with German (excuse me, with Neutsch).
A dictionary of Japanese pornography terms. After you've mastered the theory, you can read about it in greater detail, then test your knowledge. (First link SFW if no one's reading over your shoulder, others N.)