If you're using IIS 7 (Windows 2008 / Vista or higher) you could use the IIS URL Rewrite module from http://www.iis.net/download/URLRewrite
You define the rules either in web.config or through the IIS front end.
For example I use the following for friendly URLS to shop items on my site.
It makes mysite.com/shop/package-one to to mysite.com/shop/default.aspx?package=package-one
<rewrite>
<rules>
<rule name="ShopPackages" stopProcessing="true">
<match url="^shop/(.*)"/>
<conditions>
<add input="{REQUEST_FILENAME}" matchType="IsFile" negate="true"/>
</conditions>
<action type="Rewrite" url="/shop/default.aspx?package={R:1}" appendQueryString="false"/>
</rule>
</rules>
</rewrite>