First GMail, and now GBuy. Looks like prefixing products with the letter "G" is going to be a defining feature of Google.
On single-letter prefixes of the Internet age:
First there was "e", as in "e-mail", "eBay" and "evite". The "e" prefix standing for "electronic" became synonymous with all things Internet...although electronics, now that's a technology that dates from before most people alive today were born; to denote all things Internet, you really need "i", which leads us to...
Apple's "i" prefix. I never found out what that "i" stands for in iMac and iPod. Intelligent? (Anyone knows, drop me a line.)
Now "G". There are 23 more letters to go. :)