The following would go in .htaccess for the root web directory:
Options +FollowSymLinks
RewriteEngine On
RewriteRule ^([0-9A-Za-z]+)$ /test.php?id=$1 [L]

This works where test.php is the file you want to get the ID, and ([0-9A-Za-z]+) is a regular expression that matches the IDs you're passing.

For example, site.com/42 would be rewritten to site.com/test.php?id=42

Using that particular regex is handy because it matches typical shortener IDs (eg, j8E3g), but won't fuck with people trying to get to folders or files in the same directory, because it doesn't match "/" or ".".