Q:

How to search using tags - know any good examples? 

I'm working on an application where tags are structures hierarchically - so while the list of tags is organized as a tree (just like categories), the items can appear under several such tags. 
The users should be able to search using the tags - what's the best way to do that?  One of the challenges I could think of is that since an item can have several tags, users may want to use and/or search to locate items which have both tags or items which have either one.  I also don't want this to look like a query builder or a tag cloud (tag popularity is irrelevant). 
Do you know of any good examples?  

A:

I love the way Delicious handles this

this is an 'AND' only search however.  So if you want to search for 'tag1 AND tag2' OR 'tag3 AND tag4 AND tag5', you just search twice :)
I don't remember seeing a tag search that you want, but this is how I would take a crack at it,

I'm assuming your user base is fairly tech-savvy here.  Some search engines have checkboxes to filter results (images,videos, etc.),  we use it in a similar way.