The two sites I point advisees to (who are in the exploration stage as you are) are:
Transitions Abroad... http://www.transitionsabroad.com/listings/work/esl/index.shtml
...and Dave's ESL Cafe -- in particular its discussion forums: http://forums.eslcafe.com/job/index.php
(both Brazil & Vietnam are included) http://www.daveseslcafe.com/
Neither of the above sites is tied to any particular commercial provider or school (though you can get lots of information on providers through the stories, resources, and links on the TA page in particular, as well as exploring questions such as whether to get certified and the like).
To conduct any effective search -- beyond this exploration phase -- you'll really want/need to get focused on what sort of communities you're interested in working with (kids, business people, etc) and how your background / experience / training matches with those more focused notions.
If you're looking to explore connecting to English-language schools directly you can explore: http://www.eslteachersboard.com/cgi-bin/latam-schools/index.pl
(with links to many more from the header; the above points to Latin America... you can link on to Asian & Vietnamese listings as well) http://www.eslbase.com/schools/
I would suggest direct contact with schools down the road once you've educated yourself about the field a bit (and perhaps have a better sense of the sort of work you're looking for and where you'd like to go).
If you're exploring teaching English abroad I'd also suggest two books:
Teaching English Overseas by Jeff Mohamed
Teaching English Abroad by Susan Griffith
Certification is not a requirement to teach abroad though there are all sorts of reasons to get certified (or some training that sets you in front of a class if you haven't already). But it is not a requirement to find work teaching English abroad.
If, you have the chance to tutor or teach in your community do it!! It will net you some valuable experience not to mention give you a sense of whether you like the work before shipping yourself, potentially, halfway around the world...
Hope some of the above is useful!! Good luck.
Mark L Lilleleht, Program Coordinator
Go Global! @ UW-Madison http://twitter.com/Go_Globalhttp://go.global.wisc.edu/http://www.linkedin.com/groups?gid=4071801