Maps
Mr Crow's Coordinate Checker	Convert OS <> Lat/Long. Show on OS, Google, Open StreetMap.
CACH'EYE	CACH'EYE is a free personalised map service for geocachers.
OS getamap	Create your own on-line OS map.
OS Gazetteer for place names	Use place name search in OS getamap.
Google Maps	Zoomable maps focused on an address or post code
Google Earth Download	Google Earth lets you fly anywhere on Earth to view satellite imagery, maps, terrain, 3D buildings.
Open Street Map	OpenStreetMap is a free worldwide map, created by people like you.
OS National Grid	A BEGINNERS GUIDE TO GRID REFERENCES
Anagrams
Internet Anagram Server	The Best anagram solver from Wordsmith
Codes & Ciphers
Rumkin Cipher Tools	Definitive resource for encryption techniques.
Decode Telephone Pulses	Detect DTMF tones in audio files.
Huffman Coding	Generate an optimal Huffman Tree.
Huffman Coding Theory	In computer science and information theory, Huffman coding is an entropy encoding algorithm used for lossless data compression.
Codes, Ciphers, Encryption and Cryptography	Cryptography is the discipline of using codes and ciphers to encrypt a message and make it unreadable unless the recipient knows the secret to decrypt it.
Morse Code Theory	Morse code is a method of transmitting text information as a series of on-off tones, lights, or clicks that can be directly understood by a skilled listener or observer without special equipment.
Morse Code Decoder/Encoder	Rumkin's decoder/encoder.
Pigpen Cipher	The pigpen cipher (sometimes referred to as the masonic cipher, Freemason's cipher, Rosicrucian cipher, or Tic-tac-toe cipher) is a geometric simple substitution cipher which exchanges letters for symbols which are fragments of a grid.
Number Stuff
Number Gossip	Properties of Numbers.
Roman Numerals	I, V X, L, C D and M.
Numbers in Different Languages	Un, deux, trois ... Eins, zwei, drei ...
Number Base Converter	Decimal to Binary to Octal etc.
List of Prime Numbers	The first 10,000 Prime Numbers.
Digits of pi	Search 2,000,000,000 decimal digits of pi.
Generate Digits of pi	Generate your required digits of pi.
Golden Ratio (Phi) Digits	Search 2,000,000,000 decimal digits of Phi.
Check Digit	A check digit is a form of redundancy check used for error detection, the decimal equivalent of a binary parity bit.
Possible Numbering Systems
Periodic Table	The periodic table is a tabular display of the chemical elements, organized on the basis of their atomic numbers, electron configurations, and recurring chemical properties.
ASCII Codes	ASCII codes represent text in computers, communications equipment, and other devices that use text.
Unicode Characters	List of Unicode Characters
Telephone Keypad	A telephone keypad is a keypad that appears on a "Touch Tone" telephone.
Dartboard Scoring	180!
Car Country Codes	List of international vehicle registration codes.
International Dialling Codes	List of International Dialling Codes.
UK Dialling Codes	List of dialling codes in the United Kingdom.
Tenpin Bowling Scoring	X5/XXX81 etc.
Suduko	Solve Suduko Puzzles
Maths
Solving Equations 1	Systems Solver. up to 5 equations, 5 unknowns.
The Number Empire	Comprehensive set of Maths Tools.
Calculate Big Numbers	Big Number Scientific Calculator.
Right Angled Triangles	SOHCAHTOA
Circle Equations	Round and Round.
Graphics
Animated GIF Disassember	Break GIFs into frames and duration.
Make Q Codes	A pretty Q Code generator
Picture Finder	TinEye Reverse Image Search
Pixlr Online Picture Editor	The most popular online photo editor in the world!
iPiccy Online Image Editor	Edit pictures, apply photo effects, add text and even paint!
Colours
Colour Names	List of colours by name showing hex, RGB values, and hue, saturation and brightness..
Pantone Colours	Convert Pantone Colours to RGB.
RGB Colour Model	The RGB colour model is an additive colour model in which red, green, and blue light are added together in various ways to reproduce a broad array of colours. The name of the model comes from the initials of the three additive primary colours, red, green, and blue.
Setting Puzzle Caches
GBA Puzzle (Mystery) Cache FAQ	Tips on setting Puzzle Caches.
Pages Devoted to Puzzle Cache Solving
parmstro's geocaching pages	Some hints on solving puzzle caches.
Puzzle Solving 101 Series	The caches in this series will give you the skills to get out there and solve your local puzzle caches.
Tip for solving puzzle caches	What it says on the label.
Solving Puzzles Caches	I'd rather be caching.
How Do I Solve All These &#$@! Puzzle Caches?	A looooooong puzzle cache.
Puzzle Solving FUNdamentals	A PDF full of puzzle stuff.