If you are traveling with a US passport, you can come to the UK as a visitor for 6 months without needing a visa. However, if the UK immigration authorities think you are coming to work or study, or if they think you are going to stay more than 6 months, they can refuse you entry. They are only likely to do this if they have some reason to suspect that you are not a tourist. So, if you are going to have some red flag in your profile, you would want to bring things to "negate" this.
Two examples of red flags would be a one-way flight, and having recently applied for and been refused a visa to work or study. An example of something that they would want to see would be proof of sufficient funds to support your intended length of stay (this doesn't just mean cash, but can also include credit cards, funds in a bank account you can access with your debit card, etc). For people not staying in a hotel, they may want to contact the friend you are staying with to confirm your story.
You should expect some questions at immigration about your plans (what is the purpose of your visit? how long do you intend to stay? where will you be staying?). If, like most visitors, you are arriving on a round-trip ticket, and have the usual assortment of credit and debit cards, and will be able to provide the contact information of the friends you are staying with, you shouldn't have any problems.