Dropbox has been the darling of the cloud storage market. Although they only offer 2GB of storage for free, boosting that to 16GB is pretty easy if you can invite some friends to join (https://www.dropbox.com/referrals). In less than five years, Dropbox has grown to having more than 50,000 users and has been valued at anywhere between $1B and $10B depending on who you ask.
Given that growth and value, it's little wonder that larger players are taking notice and either making overtures to buy them out or launching their own rival services.
Google Drive leverages their massive investment in cloud infrastructure. Considering that GMail arrived before we even really coined the phrase "cloud computing" they have a lot of experience in delivering online services.
The word is that Google Drive will launch later this week with 5GB of free storage. While there are myriad cloud storage providers Google's big advantage is years of experience developing APIs that will make it easier for developers to make their software talk to Google Drive. That may prove to be a decisive edge. One of the problems many cloud storage providers have is getting their storage to work with tablet and smartphone apps. Google Drive might be a strategic play that further bolsters the growth of Android in the smartphone and tablet markets.
Google Drive is expected to appear at https://drive.google.com in the coming days (URL not yet active).