Extending the content administration pages I have a lot of different content-types and many of them refer to other content-types. 

In the administration it's becoming harder and harder to find a specific node, so I was thinking of writing my own theme and possible a module to make the administration easier. 

It would be nice to have a free text search box in the administration, that would let one search for nodes of specific types.
Also in the listing of found nodes it would be great to be able to add specific fields so that you could see the relation between different content-types and so on.

Right now I'm using hook_menu to add a new page with a link link for each content-type, then in the callback I collect information about all nodes using a simple db_query call and then display the result using theme('table',...). 

Can anyone point me to some tutorials on how to achive this?
I would recommend you to have a look at the Views Bulk Operations module. With VBO you can create custom views with any exposed filters and criteria you need. VBO adds to every row a checkbox which can fire different operations which you also can define on the view itself.