Simple Tags is broken with the upgrade to WordPress 2.9. Simple Tags offer many advanced tagging features, that include mass tag editing, embedded tagging, auto tags, dynamic tag clouds, and many other feature. There is a "simple" fix to make it work.

The author had hardcoded WordPress versions. Here is the original line of code, found in the file simple-tags.php under the Check version section.

if ( strpos($wp_version, '2.7') !== false || strpos($wp_version, '2.8') !== false) {

Then add a check for version 2.9 like the code below.

if ( strpos($wp_version, '2.7') !== false || strpos($wp_version, '2.8') !== false || strpos($wp_version, '2.9') !== false ) { 

Tested. Works.