Recently I faced several problems during migration from Joomla-1.5 to Joomla-2.5. One of these problems was a tag system for content items. On my 1.5 site I've used very simple Tag component. Unfortunately (as I supposed) there is no vesion of this component for Joomla 2.5.
Now I know - I was wrong! There is more powerful, simple, user friendly and well supported component! It is TorTags! It helped me to solve most significant problem of the site migration.
To developers: thank you for your work!