Virtual book tour services (a.k.a. blog tour services) connect with book bloggers and other bookish people to organize a series of online promotional events for authors across multiple sites over a set period of time. Some book tour services focus only on reviews, but others also organize book blitzes, cover reveals, interviews, and other promotional events. Check out each service's site to get an idea of what books they're looking for, what they're offering, and pricing details, if applicable.
Note for reviewers: All services on this list were looking for more reviewers the last time I checked, so if you're wanting to join a new service, these may be good options for you.