|Yahoo buys Tumblr, promises to not 'screw it up'|
|Amateur investors tap 401(k)s to buy homes|
|Tesla's fight with America's car dealers|
|Yahoo's $1.1 billion acqui-hire of David Karp|
|Death cross brewing in bond market|
Wal-Mart holds Exxon Mobil at bay and rules the Fortune 500 for the second year in a row. Which other companies made the top 50?