LevelTen has shifted almost all development to Drupal 6. Drupal 6 offers more features and more elegant ways approaching solutions. However there are still some modules that do not have stable Drupal 6 releases. Here is a list of modules we keep waiting for,

Spam flagging

Abuse (in dev)
Flag Content (in dev)
Both of these modules allow users to flag in appropriate content for moderators to review, a great tool for spam that gets through spam filters. Abuse is my perfered module in 5.x, but both work fairly well. In the mean time Flag can be configured to perform this function, but Abuse is more robust.

E-Commerce

Ubercart
Ecommerce
There are no stable releases of either of the shopping cart systems for Drupal 5. Ubercart seems to be working steadily on getting a full 6 release out, Ecommerce is moving more slowly.

Ubercart Marketplace is one we are keeping an eye on but no 6 version is even in development.

Messaging

Private Message (in dev)
The defacto module to allow member messaging boxes on your site.

Others

User Points
Meta Tags - missing the DC.Title type that is available in 5 version.
CiviCRM