While you can't go back in time, there's a fun Easter egg that lets you do just that with Google's search engine. Searching for "Google in 1998" brings you to a retro version of Google, complete with the old logo.
One big caveat: you can't actually do a new search -- it just points to a number of historic results for "Google" that link up to the Internet Archive's Wayback Machine. Back then there were only 234,000 results for the term, whereas the same search today pulls in 9.24 billion results.
(via Search Engine Land)
See also: Beyond the barrel roll: Google's Easter eggs