One of the benefits of Koha 3.0 is the availability of custom reports. Users can create and design their own reports providing the SQL in Koha 3.0 GUI format. It is a very simple to use wizard to create custom reports.
Just point your browser to /cgi-bin/koha/reports/guided_reports.pl and a wizard will ask you a series of questions and your desired report will be there.
Users can create their custom reports without wizard by entering SQL code. As Koha 3.0 has a dynamic and large active community working day and night for the development of new facilities in Koha, a few custom reports have already been designed and can be accessed here: http://wiki.koha.org/doku.php?id=sql_library
To use these custom reports SQL Code, follow the following method:
1- Point your browser to: http://[host name]:[port]/cgi-bin/koha/reports/guided_reports.pl?phase=Create+report+from+SQL&submit=Create+report+from+SQL
2- Type Report Name and Notes
3- Select type from combo box
4- Copy and paste the code
yahooooooooo
Your new customized report is ready to use.

A list of custom reports created by Koha Users is available at Koha Wiki
http://wiki.koha-community.org/wiki/SQL_Reports_Library