What tools are available to measure the email client your email marketing recipients' are using?

Not all ESPs have it, yet, I believe Litmus, Return Path & maybe Pivotal Veracity also offer it as a stand alone feature for any sender however the send their emails, you'll need to check that though. Campaign monitor were the first to produce this feature. Essentially the way that the tracking image in emails are loaded tells the ESP which email client opened the email, not unlike browsers but less clear. Form what I remember the extra information provided with the tracking image post back was not clear about the client only different for each, so you had to work out which was which.

You will be able to get this out of an ESP, but really only for web based email clients because I think they go by the domain of the email address - http://hotmail.com, http://yahoo.com, http://gmail.com, etc. This isn't possible for clients like Lotus and Outlook since the domain is unique for each business/subscriber. You might want to check out ReturnPath though. I don't think they have a tool that will give you this info, but there may be something new. They did just release a tool that provides you with numbers on what devices subscribers to your emails are reading them on. Good luck.

Yes. If you use a decent email service provider such as Campaign Monitor (or similar) this is a part of their standard reporting.