Internet

speed.cloudflare.com
My go-to "is my Internet working?" website.

Regex

regex101.com
My go-to website for understanding regular expressions.
Regex Generator
A website that excels in "reverse engineering" regular expressions, i.e. enter the string to parse and then select the parts that should be matched by the expression.

Text

URL Decoder/Encoder
A very handy website for when you don't want to think about this stuff.
UTF-8 string length & byte counter
You never know when you'll need it until you do.