Are you guys referring to the state/country government type tests or mid terms/finals type tests?
Generally, we have a few tests here (California). SAT (Standardised Aptitude Test or something like) bassically it's the normal test they give us every year starting in 2nd grade (age 7) all the way to 11th Grade (age 16-17). I never study for those and usually get well within the top 5 or 2 or whatever percentile. All multiple choice so it's rather easy even if you don't know the stuff.
Once you start Junoir High (age 12 - 13) and up till the end of High school (17 - 18) They give out Golden State Exams, which are a harder version of the SAT's, but if you pass or get high scores in X amount of subjects (6 or 7 IIRC) then you can get scholar ships around $1000. I think though you have to pay to take those anytime other than when they give them to you or something, not entirely sure.
Then we have the CAHSEE (California High School Exit Exam) which determines which tards have to stay back in high school and which can leave. Aparently, the first year they started it, they let all grades take it and the Freshmen had a %80 passing rate. Obviously this meant it was a little on the easy side, as students and parents were complaining about "why their kids should even still be in highschool if they can pass the exit test." So they've been making it progressively harder each year and only let the Sophmores through Senoirs take it now (10th - 12 grades). I still heard it was stupidly easy, so I wouldn't study for that one.
Other than those, we have normal mid-terms and finals which I ocassionaly study for (only the math and bio ones really). I don't really study for the others eaither and I usually get off doing pretty good.
I still prefer testing way more than home(course)work. Like CP said, it's just crap to give you a little practice, but mainly to keep you and your teacher busy.
EDIT:

, now that I think about it, I particularly like tests,
because it generally means the teachers don't give ut any homework.