It's hard to keep track of the worst Chrome extensions since good extensions go rogue all the time. Here are several bad Chrome extensions that you should uninstall as soon as possible, plus tips for avoiding them in the future.
Hola is an extremely popular extension for unblocking content that's not available in your region. However, unlike a proper VPN, Hola acts as a peer-to-peer proxy network. This means that everyone using Hola actually "borrows" another user's connection.
Even worse, Hola has been used as a giant botnet system. In exchange for the free service, Hola uses some of your bandwidth. In the past, Hola sold this bandwidth through the associated Luminati service. Before the company introduced tighter guidelines, malicious individuals took advantage of the system to launch DDoS attacks on major websites.