St. Louis Cardinals at Milwaukee Brewers

Cardinals and Brewers wore 1913 throwbacks.

San Diego Padres at Arizona Diamondbacks

The Diamondbacks celebrated their first ever "Throwback Game" as they celebrated the 10th Anniversary of the team's 2001 World Series Championship, leaving the Colorado Rockies as the only MLB team to have never donned a throwback uniform.

Philadelphia Phillies at Atlanta Braves

In a Negro League throwback game, the Braves dressed as the Atlanta Black Crackers, and the Phillies were again the Philadelphia Stars. The day following, the same two teams would play again in the 2011 Civil Rights game, wearing 1970s-1980s throwbacks.

Cleveland Indians at Los Angeles Angels

Tonight the Angels are wearing 1980s uniforms against Cleveland as part of their Flashback Friday 2011 50th anniversary promotion.

Chicago Cubs at Los Angeles Dodgers

The Dodgers again wore the satin 1944 night uniforms of the old Brooklyn Dodgers (and again in the afternoon), and this time the Cubs joined in on the fun, wearing their 1944 uniforms as well.

As Simply Moono writes in the comments:

The Cubs in their 1944 road greys with the red underline under a blue radially-arched "CHICAGO" wordmark. Royal Blue whole-socks with a "football-style" white oversock insert and three red
stripes.Royal Blue hat with a red wishbone "C".

Dodgers in their 1944 Brooklyn er-satin throwbacks. White raglan-stripes in the sleeves, white belt chambers and white tackle-twill numbers with a white "Brooklyn" script on the front of the jersey.
Powder-blue base jersey and pant with Dodger Blue stirrups/whole-socks mixture. Hat is Royal/Dodger Blue with a fancy "B".

Couldn't have said it better myself.

The following seven pictures are from VinScullyIsMyHomeBoy.com who sat near the Cubs bullpen. Apparently all the players (in the first five pics) were distracted, and you can see what they were looking at in the sixth picture. Read all about it here.

Boston Red Sox at Los Angeles Angels

Tonight the Angels are wearing 1980s uniforms against Boston as part of their Flashback Friday 2011 50th anniversary promotion. The Red Sox were wearing their navy alternate jerseys.

Atlanta Braves at Los Angeles Dodgers

In the first of five Dodgers' throwback games, the Dodgers wore the satin night uniforms of the 1944 Brooklyn Dodgers. Notice the "Red Sox" number of number #27 for the Dodgers. The Braves, with zipper fronts, were wearing uniforms and caps of the old Boston Braves. (although they didn't have throwback helmets)

Toronto Blue Jays at Los Angeles Angels

Tonight the Angels are wearing 1980s uniforms against Toronto in their home opener as part of their Flashback Friday 2011 50th anniversary promotion.

Chicago Cubs at San Diego Padres

For the sixth and final time of the 2010 season, the Padres went throwback, wearing their 1984 duds again. Like in the final games of the 1984 NLCS, the Cubs declined to participate.

Arizona Diamondbacks at San Diego Padres

The Padres went retro for the fifth time this year, again wearing the 1978 duds against the non-participating Arizona Diamondbacks.

Baltimore Orioles at Tampa Bay Rays

The Orioles wore all-orange, which they wore twice in 1971, while the Rays dressed as the minor league Tampa Tarpons of the mid-1970s.

Note: Special thanks to Thomas, who submitted this on the comments page, I had had the wrong date, it is corrected to August 13.

Pittsburgh Pirates at San Diego Padres

Both teams dressed in 1978 throwbacks, the Padres in the mustard and brown outfit they've worn a few times in 2010, and the Pirates in a nice looking pinstripes jersey over black pants combo with the yellow pill box hat.

Pictures are screenshots from MLB.TV and Cox 4 San Diego: