There are free practice writing tests available online. These writing tests are usually incorporated in review sites, such as MajorTests.com and TakeIELTS by BritishCouncil.org. MajorTests.com presents topics that users and reviewers could use to practice their writing, while TakeIELTS has a downloadable academic writing practice test.
MajorTests.com offers their writing practice topics primarily to students who are reviewing for their college SATs, but anyone can access the topics, along with other helpful writing resources, on their website. BritishCouncil.org offers their academic writing practice test to students who are studying for the International English Language Test System, or IELTS. However, the practice test is available to everyone. TakeIELTS also has an accompanying "model answers" file that shows good examples on how to answer the writing test.