These are tools that I have found useful for testing different things.
Some of these are used and referenced in the guides.

MD5 Hash Calculator
An MD5 hash calculator to convert a plain text string into its MD5 hash.
Very useful for checking Authenticated IMAP.
Base64 Encoder
Tool to encode strings into the simplistic Base64 encoding scheme.
Useful for checking APOP and Authenticated SMTP.
Base64 Decoder
Tool to decode a Base64 string into it's ASCII equivilent.
PHP Info
Simple function to display the phpinfo() function.