Hm, I think I got it running now. Allthrough the amounts are incorrect. For example it shows CacheCoin which I don't own anymore since 2-3 weeks.
Is it safe to provide these API keys? What can the developer of this do with it?
The Cryptsy API is generally used to grab data from the markets, or from your personal account. This data is in the JSON format, and should be parsed using a script, so it's not really meant to be human readable just by pasting the URL in a browser.
It is not safe to leave your API keys in anyone else's hands for very long. A developer can use these keys to make orders, cancel orders, and even withdrawal coins to pre-approved addresses. So in theory, your coins could be maliciously stolen/traded if you have any pre-approved coin addresses, or by trading all the coins for cheap prices in small markets where the thief has placed coordinating orders on another account.
Personally, I do not think this developer would do such a thing. But, you should assume that this site will hacked eventually, and when it does, expect everything I explained above to happen.
Here's an easy solution to prevent this from happening while still using this useful site: Do not leave your Cryptsy API enabled when you aren't using it. I enable mine from the Cryptsy>Settings whenever I am using the Import function on MyAltCoins, and then I disable it once I'm done. If you feel like taking an extra precaution, generate a new API key after every time you use your keys on any site.