Is it possible to sort tags by number of terms?

you can use printTagCloud, but it is extremely experimental and not supported right now. Tags were implemented into ZenPhoto less than a week ago. Give it time!

I know, I know, just can't wait to try everything :-)
so far its working great, check it http://www.bezbeli.ba/ I'll leave it for now :-)

Acrylian has got some pretty cool tag templates in the SVN:

printTagsAsLinks() getTags() & printAllTagsAs()

Note about printAllTagsAs(): it can print a cloud or list, by feeding it a type like this, printAllTagsAs('cloud');

Check out the function in template-functions.php

wooooorking like a charm, I only have some problems with paging the results but that may be because all the updates I did :-)
thanx aitf311

Sorry for my poor english... How must I separate the tags in the input field : space, comma, semicolon... ? Tank's for your answer.

commas