The government website for visa information would be Ministry of Foreign Affairs, but even that is not always 100% up to date. Thailand has a history of constantly changing the rules ;-)
You can also check the websites for the Thai Embassy or Consulate in your country of residence. Another good but unofficial website would be the ThiaVisa.com forum. Lots of locally knowledgeable participants (plus plenty of obnoxious ones too :-)
The current basic visa info (applicable to most G7, G8 and developed nations):
Visa Exemption entries are still 30 days by air for all, 30 days by land for G7 nations, 15 days for all others. Currently no restrictions on "visa runs" but it is up to the Immigration official if they feel you are "abusing" the system to "live" in Thailand by making regular visa runs.
Tourist visa is still a 60 day stay, but they have a new Multiple Entry Tourist Visa good for six months. The new multiple entry visa does require six months of bank statements showing a minimum amount in the account ($7000 for US citizens, maybe different in other countries). Single entry tourist visas require only a current statement showing $700 (again US).
A 30 day extension once in country is possible, but costs 1900 baht, so no cheaper than a tourist visa and can only be used once to extend your stay. Also an extension is not guaranteed to be 30 days, the Immigration official can choose to extend your stay for a shorter period. Not sure if they track your history of extensions.
All stays (visa exemption and tourist visas) expire when you leave the country and any remaining days are forfeit. So you can't use a single entry 60 day tourist visa to cover two separate 20 days stays in the Kingdom.