This may or may not be a repost from the past but I thought I would post some links that I know of.
This has helped me tremendously in refreshing my CDL knowledge.
However, you should also refer to your local states CDL Study Guide.

Feel free to add links regarding CDL written practice testing.

Crist CDL's Online Testing Center

Test-Prep-Review.Com

DMV.ORG ***Please note that DMV.ORG is not owned, operated, or affiliated with any government agency.***

Layover.com: CDL Prep Test