I just got an email from amazon, which suggested I shall buy a Wii Fit Exercise Ball. Here is the Link:

http://www.amazon.de/gp/product/B003PPEGM0/ref=snp_dp

My question is: What is this? I played Wii Fit and I have no idea how I could use this ball in the game? Does anyone have this ball or know for what it is good for? And what is the pen like thing in the picture?

I don't think you should buy it, it's a 3rd party peripheral so it's unlikely that it will be a work of genius. 

For instance I thought there was a new Wii Fit coming...

Same here.

 Notice how the product is titled:

Nintendo Wii - Fit Exercise Ball

The hyphen seperates Wii and Fit.  That means it's not Wii Fit but 2 seperate entities.  Nintendo Wii and Fit Exercise Ball. 

Intresting. But this would still mean the Fit Exercise Ball has something to do with the Nintendo Wii. I know that you can use a Fitness Ball in games like My Fitness Coach. But why does this one have spike-like things and what is the pen like thing?

The pen-like thing is a pump to inflate the ball.   I presume the spikes are meant to massage your body / stimulate bloodflow or some other BS thing.    And certainly this is... not quite false but misleading.

Read as Nintendo Wii, Fit Exercise Ball - yes there are some games (My Fitness Coach) which incorporate a fitness ball, so it is compatible with Wii, technically, but it really reads like Nintendo Wii Fit, Exercise Ball which is plain bogus.   Since there's absolutely no need for a Wii to use an exercise ball (therefore no reason for it being mentioned) this looks like a cheap attempt to cash in on Wii Fit's success.