Here are some websites that I find useful. I grouped them into different categories. Hope you find them useful too.
MacRumors Buyer’s Guide
This website keeps track of the release cycle of Apple products. You can check the average release interval of iPad, for example. You might want to check this website before purchasing one.
This website lists all the fonts available and how they look like in iOS devices, as well as they availability in a specific version of system.
This simplistic website tells you the screen resolutions of all the iPhone models.
iOS Resolution Quick Reference
Like the previous one. This website tells you more than just iPhone and more than just the resolutions of iOS devices.
This website contains lots of beautiful colors that can be used in your design.
#Reference & Tools
Time and Date
Obviously created by enthusiasts of time. If you have questions about time tone, time system, don’t forget to check it out.
Unlike Google, this search engine doesn’t filter any special characters, like ===, &, $. If the thing you are searching for contains those characters, you can try SymbolHound.
A great website, needless to say more.
I admit I haven’t visited this website many times, but it seems to be very informative.