I want to "delete" some friends from Facebook without them noticing. Is that possible? Is possible at least to hide them from Facebook chat?
If you remove someone as a friend they will not be notified, but they may notice that their friend count went down and that you are no longer on their list of friends.
To hide someone from Facebook chat, see How do I stop someone from seeing me on Facebook Chat?.
To hide their updates from your News Feed, move your mouse over one of their updates in your News Feed and click on the X that appears to the right; then click the Hide button. Alternatively, when the News Feed is set to Most Recent, scroll to the bottom of the page and click Edit Options and type their name in the Hide box.
To hide your updates from their News Feed, go to Account » Privacy Settings » Customize Settings and change Posts by me to Customize. In the Hide from these people box enter the person's name or a friend list.
You can also alter other Privacy Settings to make these people similar to non-friends. Go to Account » Privacy Settings » Customize Settings and change each setting that is set to Friends Only. Choose Customize and in the Hide from these people box enter a name or friend list. If you do not allow them to see wall posts by friends then they will not be able to post on your wall either; they may notice that if they go to your profile. They may also notice if you do not allow them to comment. If you have some of your Basic Directory Information or album settings set to Friends Only then you may want to update those settings as well. Use the Preview My Profile button and enter their name if you want to verify what they will be able to see on your profile. However, it sometimes takes some time (a few hours) for changed settings to propagate.
Unfortunately it is not possible to revoke all of their friend privileges through the Privacy Settings. For example, they will still be able to tag you in their photos unless you remove them as a friend.
If by "without them noticing" you mean that they are not notified, then you can go ahead and unfriend them. They will not be alerted in any way that you unfriended them.
But if you meant that you want them to see you as one of their friends still, then you can use friend lists to hide or show exactly what you want to specific groups of people. Here is a link about friend lists and how they work in chat. You can be "offline" for a specific group of people, while remaining as online for everyone else.
mvdavid7 lists what I would consider to be the best solutions - either deleting them or putting them on a list which you turn off in chat. If you do chose to use lists to control access, I highly recommend that you read Engadget's guide to controlling your facebook privacy using lists. It goes into a lot more detail about how to use friend lists to fine-tune your privacy settings so that only the people you want to see something can see it.
One thing to note about just blocking a user is that while they won't be notified, they will notice if they go to send you a message or poke you or whatever, because you'll no longer be on their friends list. This can become awkward if they later approach you about it and start yelling at you about how rude it was to unfriend them - yes, this has happened to me. So, I've found that creating a friends list called "Annoying people I want to go away but can't remove" OWTTE, then setting pretty much every privacy setting to deny them. This will prevent them from seeing anything or bothering you, but you'll technically still be "friends" (if you can call it that).