CAML Builder and AD Explorer

U2U CAML Builder

http://www.u2u.be/Tools/SharePointCamlQueryBuilder.aspx

CAML Designer

http://karinebosch.wordpress.com/my-articles/caml-designer/

AD Explorer:

http://technet.microsoft.com/en-us/sysinternals/bb963907.aspx

ULS Logs:

http://archive.msdn.microsoft.com/ULSViewer

XML Tool Box:
Generates XPATH:
http://www.xmltoolbox.com/index.html