Recently I was asked to check our application that we are currently developing for security issues. There were two reasons why exactly I was asked to do this:
1. I have the best experience in our team (bad one) regarding security-based issues.
2. I do really like this part of application development!

By mentioning my bad experience I mean that I've had a time when one of my projects had been hacked. I had to spend a lot of time to fix all those issues and make it not to happen again. Luckily - it didn't happen yet :)

So coming back the article itself and it's title... :)
Check this site out!
http://www.owasp.org - "The Open Web Application Security Project (OWASP) is a worldwide free and open community focused on improving the security of application software."
It's the best site regarding application security that I've ever seen. The most valuable page to start with checking your site (for today) is: http://www.owasp.org/index.php/Top_10_2007
It explains, gives examples and possible security actions against these vulnerabilities. But anyway, you shouldn't take care only about this Top 10. By browsing this site you will find a lot more less dangerous (by their classification) but yet not harmless attack vectors.