Here you will find a library of some very useful & difficult SQL scripts.

1. http://stackoverflow.com/questions/197369/library-of-useful-difficult-sql-scripts
This a StackOverFlow article which has some of the best links.  A must visit for all SQL guys...

http://wiki.lessthandot.com/index.php/Category:Microsoft_SQL_Server - This is by far the best site I have found for SQL Server scripts.
2. http://wiki.lessthandot.com/index.php/SQL_Server_Programming_Hacks_-_100%2B_List
3. http://wiki.lessthandot.com/index.php/SQL_Server_Admin_Hacks
4. http://vyaskn.tripod.com/code.htm
5. http://www.nigelrivett.net/
6. http://www.paragoncorporation.com/ArticleDetail.aspx?ArticleID=27
7. http://stackoverflow.com/questions/197111/fastest-get-duplicates-sql-script 
8. http://bit.ly/aHoHxD - Joe Celko's SQL puzzles & answers ebook


I will keep on adding links to this list & if anyone of you have some useful links then feel free to share it here by adding comments, you can even ask any Q's regarding SQL here.

So keep scripting....