When I am testing web applications, I like to analyze the system to find areas of testability, and to identify potential areas of instability. To help remember areas to investigate, I use this mnemonic:

FP DICTUMM

F – Framework (Struts, Rails, Plone, Spring, .NET, etc.)
P – Persistence (Hibernate, Toplink, iBatis, Active Record, DAOs, etc.)

D – Data (database, flat files, etc.)
I – Interfaces (code level, web services, user, etc.)
C – Communication (HTTP(S), web services, SOAP, XMLHTTPRequest, database drivers, etc.)
T – Technology (.NET, J2EE, Ruby, PHP, Perl etc.)
U – Users (Human end users, and other systems or apps that use this system)
M – Messaging (JMS, Enterprise Message Beans, Web Methods, etc.)
M – Markup (HTML, CSS, DHTML, XML, XHTML)

Each of these areas help inform my testing, or provide areas of testability. Understanding the whole picture from the data model up to the HTML displayed in a web browser is important for my testing.