Useful Links
https://gamefaqs.gamespot.com/
Great place to find game guides, finding almost all you could want to know about game content, find cheat codes, for development utility and also for making proper protections against the codes.
https://www.vgmaps.com/
Thousands of maps/screenshots of your favourite video games.
Cumulative Probability Calculator
When working with achievements with an RNG factor you will often want to know the probability of success for players given a number of attempts. (see unwelcome concepts, achievements with low odds). Use it to calculate P(X>=1), assuming the player needs to succeed only once across several attempts, to estimate how many attempts an might require for less lucky players.
https://www.zapstudio.net/framecalc/
Simple utility to calculate frames out of time given and the other way around.
https://www.romhacking.net
The biggest repository of patches to hack ROMs.
https://emn178.github.io/online-tools/crc32_checksum.html
Drag and drop checksum checker utility. You can also get the checksum from the rom you have loaded from the retroachievements menu in the emulators.
http://www.marcrobledo.com/RomPatcher.js
An online ROM patcher utility.
http://jsonviewer.stack.hu
Excellent for viewing raw achievements
http://gamehacking.org
Provides many Pro Action, Game Genie, and Game Shark codes which can be converted to RAW addresses on the site.