ASCII table with hex, decimal, octal and binary

The table you end up wanting three times an hour. Beyond lookups, the structure of ASCII is itself useful: the relationships between ranges are exactly what several classes of challenge exploit.

It is worth treating as more than a lookup table because ASCII was designed, not accumulated. The ranges line up on bit boundaries deliberately, and several standard puzzle mechanics are just that design being exploited - which means the table doubles as a list of tricks.

Structure worth memorising

• Uppercase and lowercase differ by one bit - 0x20. A is 0x41, a is 0x61. XORing a letter with 0x20 toggles its case, which is why case-flipping appears so often in XOR challenges.
• Digits start at 0x30 - so '7' - '0' is 7, the conversion every parser performs.
• Control characters are 0x00 to 0x1F - printable output containing these means you decoded with the wrong scheme.
• 0x7F is DEL - the odd one out at the end, not a printable character.