$_GET works fine, but you have to use a MODX link tag to create the links (which you should be doing for all links anyway).
If you're sending them to the page with a resource ID (in parentheses in the Resource tree) of 12, for example:
<a href = "[[~12? &propertyName=`value`]]"> Link Text</a>
Make sure the value is in back-ticks, not single quotes.
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PLEASE, PLEASE specify the version of MODX you are using.
MODX info for everyone:
http://bobsguides.com/modx.html
