This may seem a bit paranoid, but I read an interesting post today at Coding Horror: A Question of Programming Ethics. Basically, somebody wrote a shareware program called "G-Archiver" that will store a backup copy of your GMail messages on your hard drive; in order for this to work, you obviously have to provide your password. However, it turns out that the program was emailing all these passwords back to the programmer. Oops.
In fairness, you need to type your password into your computer somehow if you want to get at your email; this could be through a web browser or a dedicated email application (e.g. Outlook Express). So, you have to make the trade-off: who do you trust? Personally, I'm willing to trust Microsoft applications, although I know that other people disagree. I'm also willing to trust Firefox. However, open source isn't a panacea; just because something can be read, that doesn't mean that anyone has actually read it, particularly if it's obscure. It's also worth mentioning that the same thing could be done on other platforms (e.g. a Mac); this isn't a virus, it's the program doing exactly what it was designed to do.