SkyWarn Spotter Training--Two courses, no cost. Can
be taken online, but some areas may require that you attend in-person training (also free) in order to be certified.
Universal Class Meteorology Class--Costs
about $100 USD, but if you have a library card, check your library's web site. Mine offers free unlimited access to Universal Class.
Udemy Meteorology Courses--Check frequently; as
of this posting, all courses are $14 USD but are usually much higher. They do offer discounts fairly often.
JetStream--NOAA's online "school" for meteorology.
NWS/NOAA Training Courses--Meant
for forecaster training; but as far as I can tell, they're free for anyone to take online. Includes Radar & Applications,
Warning Operations, and more.