OWASP Released Top 10 Web application vulnerabilities for 2007
http://www.owasp.org/index.php/Top_10_2007
Here u can find the Top 10 Web application vulnerabilities for 2007.
as usual, the most frequent attacks are Cross Site Scripting, an SQL Injections...
I highly recommend to inspect the summery list for the vulnerabilities to learn how to avoid them.
Scroll down for the summery list represents the application vulnerabilities and explaination links for each of the them.