Google has come out with a handy URL based API for adding mathematical formulas to html documents. I haven't seen a GUI tester for this yet, so I'm adding one here.
All formula image links start with the address
The code that follows the link describes the formula to be displayed. For example, the tester on this page loads with the quadratic formula. Documentation on the TeX codes used to create formulas can be found on Wikipedia at:
To use the tester, place your formula tags in the text field labeled 'Formula'. Then click, 'Test Formula' to see the formula as it will be displayed. The 'Formula URL' box will contain your formula encoded into the proper web address.