This obviously is not an exhaustive list of everything, but I tend to use it as a reference point for things that are hard to search for that I want to jot down somewhere.

Floating Point Converter

A fantastic tool to show you how floating point numbers will be represented, and if they can be represented exactly.

PGP Path Builder

For building a trust path between two keys.

Android Census

Exposes a ton of information across Android devices including sysctls, files, libraries...

Mike's Toolbox Enhanced Multi-Threaded SSL/TLS Test Server

A nice site that exposes nearly everything about your client's SSL session: ciphers, extensions, and secrets.

mod_rewrite Rule Tester

This lets you put in a series of potentially complex mod_rewrite rules and then test arbitrary urls against them.

CA Risk Incident History

A wiki page that lists security incidents of Certificate Authorities.

File Formats

A good resource for common file formats, including zip, jpg, and gz.

The Bit Twiddler

A collection of bit twiddling hacks to do constant time things such as finding if a number is a power of 2 or not.

Consolidated Terminology for Anonymity

When you're confronted with a term you've not seen before but you know means something specific (like 'unlinkability'), this may help.

Client Certificate Viewer

Used to get information about the client certificates you can send to a server, and how the values are mapped to variables.

Unicode Spoofer, the ICU Project, and SpoofChecker

Tools for generating or checking for Unicode spoofing attacks

oid-info & alvestrand.no

If you need to look up OID information, these databases are a good place to start. (Here's how to get your own OID arc)

Fixing a Git Messup

An awesome flowchart on how to fix your git messups (of which I have many, and man git provides no assistance).

x86 Oddities

Interesting and strange things in assembly.

Domestic Islamic-Related Terrorism in the US, 9/11/01-6/11

Have you ever wondered just what we're being protected from? Here's the list, with categories showing how sophisticated the terrorists were.

John the Ripper Hash Formats

I spent a lot of time working with john, and figuring out what hash formats looked like was a big pain in the butt. This guy did it for you.

Converting GPG keys to subkeys

Almost no one (including me for now...) uses PGP keys the correct way - keeping on non-expiring offline key for signing individual subkeys over time. This is a guide on how to set that up.

Tools Library

A library of Pen Testing and Reverse Engineering tools, with categories and descriptions.

Windows Kernel Structures Over Time

Comparison of Windows Structures diffed between OS and SP over time.

Windows Security Log Index

Windows Security logs, with documentation about them. Might be findable with google, but here for posterity.

Linux Hardware to Kernel Modules Database

Paste the output of lspci -n, and it tells what kernel modules you should compile in. Invaluable!

RegExr

The only way to write a regular expression. Once you try this you'll never go back.

Daily DNS Changes

For watching the internet change.

Hardware Poster

For identifying what part it is that you have.

Freebase

A database of everything (or aiming for it) in a more structured format than Wikipedia. You can actually query Freebase and pull out specific data in reliable data types. As any community maintained project, there may be holes. But still, a great resource. It's data enabled the Wal-mart Expansion video by providing founding dates for all Wal-mart stores, and latitude and longitude of all Us Zip Codes.

MySQL Development Tools

I'm tired of using phpMyAdmin to browse tables and run queries when SQL Server Management Studio is so nice. I'm going to use one of these next time.

Online UML/Database Schema Creator

Just like you'd draw on the board, or in Visio, you can do it hear, and then export the actual MySQL commands. I haven't used it much, but it looks super handy.

Jeffrey's Exif Viewer

View EXIF data for any image, either by upload or URL.

Solutions for tracing UNIX applications

How to examine, trace, and debug out of control or hung processes .

IP Index Encyclopedia

If you get hammered by IPs all the time, this can quickly help you track it down. Schools and their pesky students are the easiest-to-find offenders.

Undersea Cable Map

A map showing all the undersea cables, and their termination points.