Hat Lek (also spelt Haad Lek, Had Lek, Hadlek, Hatlek) is the location of the southern-most border crossing between Thailand and Cambodia. Its Cambodian counterpart is Cham Yeam, about 8 km from Koh Kong.
Hat Lek is the name of the border. The only things here are a few shacks selling food and drinks and possibly a guesthouse. If you're desperate for a break from border hassles, head 15 km further to Ban Khlong Yai, which has nice seafood but no beach.
Minibuses connect the border with Trat, departing every 45 minutes between 06:00 and 18:00, taking between 1.5 and 2 hours, 120 baht. A cheaper and more breezy option involves songthaews (pickup trucks that act as local buses). However, the journey to or from Trat may involve changing between songthaews in Ban Khlong Yai (80 baht), to another songthaews the remaining 15 km to the border (20 baht). Songthaew price rigging at the border on may put you off, with some unsavoury-looking character inflating the price and taking a cut from the fare before passing on a fraction of your money to the driver. If such an arrangement upsets you, head away from the immediate border area and try to negotiate directly with a passing driver.
There are also more sinister overpriced transport scams at the border if entering from Cambodia. Transfers to Trat costing 1,400 baht have been forced upon some unfortunate travellers. If someone prevents you from taking either a minibus or songthaew, walk away. If they persist, head away from the immediate border area, head up the hill for about 150 m to the police checkpoint and then pick up a songthaew or minibus from there.
Buses serving Trat depart from either the Eastern Bus Terminal (Ekamai) or the Northern Bus Terminal (Mo Chit). Day-time direct 1st class buses from Ekamai (~240 baht) average a bit more than 5 hours, while the less frequent services from Mo Chit take a little less; late-night services are quicker still. While most 2nd class buses (188 baht, air-con) take a bit longer as they may make more stops and use slower and less direct routes. The midnight departure from Ekamai takes only 4.5 hours.
Head across the border to Koh Kong, 8 km away. The border opens at 06:00 and closes at 22:00. The ferry to Sihanoukville from Koh Kong no longer runs. Onward travel from Koh Kong to Sihanoukville and Phnom Penh is by full-sized air conditioned bus which leaves from Street 3 at around 08:00 daily.
There are ATMs dispensing baht. Money changers abound.