The most famous German athlete on US professional teams is Dirk Nowitzki (Wiki Info auf Deutsch) and now, there's another German basketball player: Dennis Schroeder (Atlanta Hawks), and Tibor Pleiss playing for the Utah Jazz farm team in Idaho. And a number of hockey players (on Farm teams) and a few football players too.

1) NHL players born in Germany - Liste der deutschen Spieler in der NHL
NHL-Site in German (Link on the yahoo-NHL site (on top))

New York Islanders: Thomas Greiss (Goalie)
Anaheim Ducks: Korbinian Holzer (Defenseman)
Boston Bruins: Dennis Seidenberg
- Arizona: Tobias Rieder - Edmonton Oilers: Leon Draisaitl (since 2014-15)

2) Germany National Basketball Team (also lists the ethnic background of some German players) - NBA and ABA players born in Germany
NBA (Wikipaedia auf Deutsch)

3) 2013: In the coming season, there will be 3 German football players in the NFL - 2014: 4 players
More links: New England Patriots, Sebastian Vollmer agree to terms on new contract (2014 - still playing)
- New York Giants: Markus Kuhn (deutsche Info) - Indianapolis: Bjoern Werner (Linebacker) (deutsche Info) - New Orleans Saints: Kasim Edebali - Linebacker (deutsche Info)
NFL players born in Germany - German players of American Football (Uwe von Schamann - a former kicker) / (4 Deutsche in der NFL - 2011)

4) and the first homerun by Donald Lutz ("Der 24 Jahre alte Outfielder der Cincinnati Reds") in Major League Baseball (Artikel - still playing in 2014) -
and a second player: Nadir Ljatifi (also signing with the Reds)