Depending on what editor you use, it can often be tricky to remember or find certain mathematical symbols.
The site below allows you to simply draw the symbol you need and the code will appear next to a list of symbols closely matching your drawing.
If you're not much of an artist, a full list of every symbol available, including obscure ones can be found here:
Sometimes you want to quickly view a line of equations to check if it looks OK and that the code works. Usually this involves having to compile the whole document which can be very time consuming.
There are actually plenty of equation editors online where you can simply type in your code and the resulting formulae will appear immeditely. The site below is just one of these sites and includes some symbols as well, which can be clicked on to use. Any code you type in can be copied across to/from your TeX file as well.