Humber, who had his option for 2014 declined, Laird and Pagnozzi can all elect free agency due to prior outrights, and Clark will become a Minor League free agent following the conclusion of the World Series.
Valdes, 35, made 17 appearances, including one start, in two stints with the Phillies this season and went 1-1 with a 7.46 ERA. He was 4-5 with a 2.86 ERA in 14 starts at Triple-A Lehigh Valley and held Major League left-handed hitters to a .229 batting average.
Valdes has pitched in parts of four Major League seasons with the Mets (2010), Cardinals (2011), Yankees (2011) and Phillies (2012-13) and owns a 4.94 ERA in 95 appearances.
Singleton, 22, who entered last season as the club's No. 2 prospect, according to MLB.com, hit a combined .230 this year in 90 Minor League games across three levels, with 17 doubles, 11 homers, 44 RBIs, 59 walks and a .351 on-base percentage.
He missed the first 50 games of the season because of a suspension following his second positive drug test. He played his last 73 games with Triple-A Oklahoma City and finished by hitting .250 with 19 RBIs and 22 walks for a .400 on-base percentage in August.
Humber, who had a $3 million option for 2014, appeared in 17 games, including starts at the beginning of the year, in which he went 0-7 with an 8.82 ERA. The Astros took a low-risk gamble that the former Rice University star would thrive in the city in which he played college ball, but it didn't work out.
Humber said last week he wants to continue to pitch.
"I've showed flashes of being as good as I've ever been," Humber said. "I'm not hurt. I still enjoy coming to the park and I'm still relatively young . It's one of those things that's like, 'Man, you walk away from it, are you going to look back and think I wish I could have kept going?' There's times in the past, I didn't know if it was going to work out. It's still fun. It beats working, you know?"
Clark, 33, appeared in 16 games for the Astros last season, making 10 starts behind the plate. Thrust into the Majors when the Astros lost backup catchers Carlos Corporan and Max Stassi in a span of three days due to concussions, Clark provided one of the year's feel-good stories when he broke an 0-for-25 slump to start his career with a single.
Laird, 26, had two stints with Houston last season, appearing in 25 games. He had five homers and 11 RBIs, and made 16 starts in what was his second season with the organization. Laird was originally claimed off waivers from the Yankees in September 2012.
Pagnozzi, 25, was acquired from Atlanta in exchange for cash considerations on Sept. 3 as the Astros were forced to add catching depth when starter Jason Castro hurt his knee. Pagnozzi appeared in nine games for Houston, making six starts.