Well, there are only few other ways to hack someone, without making him to download an infected application or file. I'll try to explain the stalking method..
Stalking is one of the easiest methods to gain some info on the person, and some of that info can be obviously used to reset his accounts on various Internet communities, social network sites, etc, etc.
First thing you'd have to do is to lookup his username on Google, if search returns no results, you'd have to get his mail address, after you get his mail address you'll look it up on social network sites, if he has an profile on Facebook or any other social network site, you have his first and last name now, which will help you to determine where he lives, does he have profiles on other sites, was he mentioned on some news portals, etc, etc.
Some of the social network sites display DOB (date of birth) to guests, and some don't. In order to get his DOB on Facebook, you'd have to add him as a friend via some fake account, chances to be accepted as a friend when you don't have a profile picture are quite low, so you should make it to look like a profile that's used quite often. Let's say he accepted you as a friend, you can see his Facebook profile now, and DOB, you can go and try Password reset for his mail account, now here's the tricky part.
Security questions, most of the people use stupid security questions, such as:
- Name of the pet
- Mother's birthplace
- DOB of the boyfriend/girlfriend
- Favorite movie
- Favorite song
You can get all of that by checking what they post on Internet communities, while you can get name of their pet by checking their album on Facebook, as most of them have pictures of their pets.
Things you have learned from this post are: don't use such security questions, don't register Facebook profile under mail address that you're using for Internet communities, don't use same username everywhere, etc.
That's all