What's the difference between the ISEE and SSAT?
The ISEE and the SSAT are published by two different companies.  The
SSAT is the more common of the tests.  The SSAT comes in two
versions- one for grades 5-7 and one for 8th grade and up.  The ISEE
has three versions- Lower (grades 4-5), Middle(grades 6-7), and Upper
(grades 8-11).  Since there is a separate version just for them, younger
students tend to take the ISEE instead of the SSAT.  
Internet Resources
Educational Records Bureau - This is the publisher of the ISEE test.  
You can use this site to register for the ISEE.  

Secondary Schools Admission Test - This site allows you to register for
the SSAT.  It also allows you to search for schools.  A great feature of
this site is that it posts a list of schools that are currently accepting
applications in case you are trying to make a mid-year transition or got a
late start on the application process.  
The only practice ISEE tests put
out by the ERB:
The only practice SSAT practice
test put out by the people who
write the SSAT:
The best prep book that I have found:
(The SSAT/ISEE books tend to much
lower quality than the SAT books)
Christa Abbott, M.Ed.