Grendel-Scan
 
  Below are links to other resources for testing web application security.


Documentation
I highly recomend The Web Application Hacker's Handbook, especially for someone just starting out with web app security.

OWASP has a number of resources, but some of them need more work. They also have local chapter meetings.

SQL Injection Cheatsheet

XSS Cheatsheet


Tools
Mark Woan has written a number of useful tools for web app testing that don't get much attention.

Absinthe is a good tool for automatic extraction of data through SQL injection.

SQLMap is another good data extraction tool. It has more features than Absinthe, but can be harder to use.

There are a ton of Firefox plugins that I use for testing. Most of them are geared at developers, but are also very useful for manual application penetration tests. For the record, I don't have anything against IE, it just doesn't have the same selection of plugins.

Add N Edit Cookies
Cert Viewer Plus
Console2
CookieSafe
CookieSwap
CustomizeGoogle - Helps a little with search-engine recon
DOM Inspector
Firebug
FoxyProxy - Extreemly useful for testing with an intercepting proxy
JavaScript Options
JSView
Live HTTP Headers
Search Marker
Uppity
User Agent Switcher
Web Developer


Other intercepting proxies (Grendel does this too)
Burp
Charles Proxy - Has some useful Flash features
Fiddler
Paros
Webscarab - My favorite before I wrote Grendel


Other open source web app scanners
Nikto - Isn't quite the same because it is designed to find existing vulnerabilities. Grendel can run the tests found in the Nikto database.
W3af
Wikto - Very similar to Nikto, but with a few more features


Commercial web app scanners
Acunetix
AppScan
N-Stalker
NTOSpider
WebInspect