Disclamer: I have been working on this for several months, it is a work in progress. I want to go ahead and post it, but let you know that it will always be a work in progress - much like our children and spouses. If you have something to add (or spot an error) please email me. Then check back often to see any updates :)
Gulf Coast Christian Home Scholars
Pearland Homeschool Alliance
FreeHomeschooling101 is an excellent resource for assessment tests that cover grades K to 12. These tests are available for multiple educational areas such as math, spelling, reading, science and many more.
One of the more prominent links featured by FreeHomeschooling101 will lead you to Internet4classrooms. This site boasts an invaluable stash of assessment exams, particularly for high school students.
You can also take a look at the free practice assessment tests offered by Polyweb.net. Do note, however, that these tests are geared towards grade levels of three and above and will not be the best resources for beginning homeschoolers.
Parents of Christian homeschoolers can take advantage of the assessment test directory provided by ChristianHomeschoolers.com.
MINDsprinting offers some very comprehensive assessment tests for all grade levels in the subjects of reading and math. Be aware that MINDsprinting is a private tutoring program, though you do not have to be enrolled as a student to make use of the assessment resources. These tests can be taken multiple times for no cost. MINDsprinting boasts four pages of assessment results for each test, which should give homeschooling parents a fairly decent idea of their child's reading and math prowess.
HomeschoolMath.net provides a directory of links for math-specific assessment tests. Do note, however, that not all of these site suggestions offer free testing, so you will want to read the descriptions of each proposed site carefully. The latter portion of this directory deals with SAT and ACT testing, resources which may be practical for more advanced students that are in the college prep stage.