What visa options are available to me before I enter Thailand?

To say that Thailand’s visa regulations are extraordinarily complex just doesn’t do things justice. And if anything they seem to get more and more complicated as time goes by.
Well there are three main options and these themselves have options within them.

On-entry Visa: Most nationalities can receive 30 days on entry automatically with the option (at the Immigration officers discretion) to extend by 10 days (OR SOMETIMES 14 DAYS at a cost of 1,900 Baht) at an Immigration office. Go here for the list of countries: http://www.imm3.police.go.th/eng/
Update 10th December 2008 - Please note that the 30-day ‘on entry’ visa is now granted only to those entering the country via an airport. Those who enter Thailand via a land border will receive only 15 days.
There are countless foreigners who have lived in Thailand for years and simply toddled off to a border point every 30 days, done a quick ‘in and out’ and received a fresh 30-day stamp. That all changed in late 2008 when the 30-day stamp was cut to 15 days. This was obviously a move to get rid of the ‘foreign riff-raff’ because 15 days is a ridiculously short window and surely no one can be bothered to do border hops that often.

Tourist Visas: Can and must be issued by a Thai Embassy or Consulate and there are different numbers of entries (not always easy to get more than one though!). A single tourist Visa would automatically entitle you to 60 days on entry to Thailand, with the option to extend by 30 days (again at 1,900 Baht) at an Immigration office. A double entry tourist Visa would entitle you to the above, with a further 60 day entry once you’ve left and returned (exit and re-entry) to Thailand (see border runs further on) and the option to extend by a further 30 days. A triple entry would be the same but with THREE entries of 60 days etc. The most entries I’m aware of is four, but you’ll find that most neighboring Asian countries will only issue singles or doubles at best. A common question is ‘which embassy or consulate is the best to go to for a tourist visa? Penang in Malaysia? What about Kuala Lumpur? How about Vientiane in Laos? Or Cambodia even? The answer is ‘who the hell knows?’ You hear just as many success stories as you do refusals - from all consulates. It all really depends on which way the wind is blowing.

So let’s recap on what you can do with a double-entry tourist visa if you’re lucky enough to get one.
Let’s say you enter Thailand on the 1st January (for sake of argument). You get 60 days on entry which will allow you to stay in Thailand until the 28/29th February. Just before the 60 days expire (or on the last day itself) you go to immigration office in Thailand and extend for a further month. Then you can extend for another two weeks (all extensions are currently 1,900 baht). When your final day of your final extension is almost upon you, you catch a bus, train or plane to a neighboring country’s border point and then turn around and re-enter Thailand. Thus you now activate your second entry and your second 60-days. So in effect, a double entry tourist visa will get you 60 days plus 60 days plus an optional extension of one month on each entry and then a further extension of two weeks on each entry (all extensions can be done at an office within Thailand). So you would get about seven months out of your double entry tourist, but that includes one border hop and FOUR trips to the immigration office!
For the cost of tourist Visas in the UK go here: http://www.thaiconsul-uk.com
Remember - you cannot work legally in Thailand on a tourist visa!