Beware of applications or plugins of somewhat bad programmers.
Jeff Atwood of Coding Horror has written up a story of a G Mail password theft that will retrieve all stored password data of users of an G Mail application called G-Archiver.

Read Write Web picked that up and asked some interesting questions such as
How often have you given your usernames and passwords to various services, including webmail, to a new application you want to check out?

Read both articles AND the comments, to get more insights how to prevent security holes.

Follow up of security articles.