Myawaddy (also sometimes spelt Myawaddi, Myawadi or Myawady) is a border town in Myanmar. Access for foreigners is currently only possible from Mae Sot in Tak Province, Thailand, though this is set to change in a few years time.
At some point this is expected to be a full international border crossing as part of the Asian Highway Network route AH1 which will connect Tokyo in Japan with the border between Turkey and Bulgaria. At that time the Mae Sot - Myawaddy border crossing will probably become the main overland entry point for foreign travellers wishing to get from Bangkok to Yangon (and vice-versa) overland.
Currently however foreigners may only enter on foot, are not allowed to travel further overland into Myanmar from Myawaddy, and are not permitted to stay in Myawaddy overnight, hence they must head back to Mae Sot before the border closes at 18:00.
Despite these restrictions, the Myawaddy - Mae Sot border crossing is a moderately popular destination for tourists and expats in Thailand who want to make a "visa run" (i.e. get a new Thai entry permit stamp by making a short visit to another country).
The time in Myawaddy is 30 minutes earlier than in Mae Sot, and the traffic (mostly) drives on the right.
Get in 
Foreigners can only cross the border into Myanmar at this point on foot (unusually, foreigners are not even permitted to cross with bicycles).
From the Thai town of Mae Sot, take a songthaew or other transport to the border crossing at the Thai-Myanmar Friendship Bridge over the Moei River; Myawaddy is immediately on the other side of the bridge.
Foreigners must deposit their passports at the Myanmar immigration checkpoint. The officers ask for 500 baht, whereas officially the fee is $10. They will haggle and mess around, using a variety of ruses, such as your US dollar bills being printed in the wrong year. To save (their) face, carry a selection of notes, or just be patient - the fee is $10. No fees are payable at the Thai checkpoint. Normal exit and entry stamps are issued at both checkpoints.
Currently (March 2010) it seems to be impossible to pay $10 insteard of 500 baht. When asking for an explanation of why the border in Mae Sai charges $10 and the border in Mae Sot 500 Baht (which is clearly more than $10) guards may claim that these are different border crossings and there are more foreigners in Mae Sai so it makes it easier to exchange the dollars, or something like this. In any case, just be prepared to pay 500 baht. Even if you pretend to have no thai currency with you, the officers then let you enter the town without taking your dollars but tell you to change them into baht there and pay 500 baht on your way back!
As of late March 2010, on your re-entry back into Thailand you are given visa for only fifteen days, not "30 days" (as a certain guidebook wrongly says).
While in Myawaddy the Thai Baht is universally accepted, as such, there is no need to change your Baht into the Kyat (the Burmese currency).
Get around 
Myawaddy is centred around the wide strip of highway that leads from the Friendship Bridge through town and is small enough to be explored easily on foot; alternatively, ubiquitous red bicycle rickshaws go everywhere.
Places of interest in town include:
The Golden pagoda, 2-3 minutes from the border checkpoint by rickshaw. Very picturesque site with several big Buddha images enshrined on its grounds, one of which is a replica of the golden Buddha image of Mahamuni temple in Mandalay. Locals claim that this pagoda is more than 1000 years old.
The Crocodile temple, 6-7 minutes from the border checkpoint by rickshaw. Very unusual temple situated on the back of a gigantic crocodile statue.
Three more temples with big padodas a bit away from the town center in neighbouring villages (actually there are many more pagodas in surrounding areas, as everywhere in Myanmar).
One of these pagodas is situated on the grounds of the children's dancing school, so you can see many small kids there. There are also more than a hundred similar sitting Buddha images on one of the sides of this pagoda.
Another temple is situated on a hill 100 metres walk from the previous one. It also has a big pagoda and gives a good view of the surrounding area. Visiting these distant places gives a bit of experience of "real Myanmar" with small wooden huts and locals working in their fields.
There are several guesthouses and small hotels, however foreigners are currently not permitted to stay in Myawaddy overnight.