When friends ask me about visa runs I usually refer them to a company called Jack Golf. Their service is usually good!
To Cambodia for visa stamps upon arrival (30 days):
Daily take off (gathering) is at Sukhumvit Road 12. The time: 5:30am and 7:00am.
You need a pasphoto and 2000Baht. You will be back in Bangkok the same day around 3pm.
To Laos for a tourist visa (2 months):
This bus leaves around 7pm. You need two pasphotos and 6600Baht. A long ride! You have to stay overnight in Laos.
Make sure you call them before hand: 02-2511946! to find out what day they depart.
Another more comfortable option is a two hour flight for a tourist visa in Malaysia, city Penang . Book it yourself via Air asia or the agents in the newspaper.
There are many visa run companies in the Bangkok Post newspaper with good services!
For good update information check: http://www.thaivisa.com/