A: 
RewriteRule ^([^\.\?]+)/?(\?.*)?$ /$1.aspx$2 [L]
Darin Dimitrov
A: 

What version of ISAPI_Rewrite you are using - 2 or 3.

The v3 syntax is:

RewriteBase /
RewriteCond %{REQUEST_URI}.aspx -f
RewriteRule ^([^/.]+)$ /$1.aspx [L]
TonyCool