Below are some useful SE sites I've come across. I'm sure there are many more so please reply and add to the list. These sites are a great place to find custom macros that help speed up your design workflow.

http://barhamsoftware.com/SE_Tools/index.htm
http://www.geocities.com/segurucool/
http://www.solid-tools.com/

If you are interested in custom programming of SE then the site below should definetly be on your list:

http://www.jasonnewell.net/portal/

The next ones are a combination of German and English. There are free translators available on the web to help read what is available:

http://www.incobase.de/downidx_cad_se.html
http://www.cadcam-consult.com/wh_main/index.html