Do organisms exist that are able to live indefinitely if they are not killed by external factors?
Under external factors I would consider things like predators and natural disaster but not illness by bacteria f.e.
Yes. The Bristlecone Pine, Pinus longaeva, is one example. This species boasts the oldest individual living organisms, and also has been convincingly argued by Lanner and Connor (2001) to show no evidence of senescence.
While the Wikipedia page on Biological Immortality (as of June 2013) unfortunately ignores plants, the pages on Negligible Senescence and Longest-lived Organisms list many plant seeds, clonal groups, and individuals.
I now found this Wikipedia article on biological immortality. It's pretty much what I was looking for.
Wikipedia describes the phenomenon as follows:
It has a list of such organisms as well, consisting of
Addendum: This blog post takes a good look into the myth of lobster immortality. It seems to be as much a myth as the result of any substantial observation.