Only protecting email with a password
There are a few apps out there that claim to be able to do this. iLock (http://www.osxbytes.com/ilock.html) and Mac App Blocker (http://knewsense.com/macappblocker) are two, though not free.

Creating a non-admin account as Patrix suggested, and only using that during the party is the best solution since party-goers won't be able to see anything of yours (mail, browser history, files, etc). You'd just need to share your music with that non-admin account (this describes the process: http://www.cnet.com/how-to/how-to-share-itunes-libraries-between-user-accounts-in-os-x/).

If you want to use your account, the most effective way would be to turn on Parental Controls in your Settings, and only enable iTunes and maybe a browser. However this won't work if your account is an administrator account.

If your account is an admin account, and you want a free method of preventing someone from seeing your mail (I assuming you're using the OS X Mail app), you could rename your mail folder temporarily (Library/Mail) and then change your email password temporarily so that if someone opens the Mail app, they'll see nothing and it won't download any new mail. If you're paranoid, you could move the mail folder to an encrypted volume (created in Disk Utility) or a removable drive. But if you're that worried, using a different account for the party is easier and safer.