With a month remaining in the regular season, the group of impending free agents have about 30 games left to leave a positive impression on the rest of baseball.

Some have been superb since the start of the season, while others have used a second-half surge to show teams what kind of impact they can have going forward.

Here's a look at 10 players (listed alphabetically) who have helped their own causes in recent weeks and/or months:

Madison Bumgarner, LHP

The left-hander's first half (5-7, 4.03 ERA) was underwhelming, causing some to wonder whether Bumgarner's best days were behind him at the age of 30. In his nine starts since the All-Star break, the southpaw is 3-1 with a 3.10 ERA, a 0.91 WHIP and he's allowed only six homers in 58 innings after serving up 17 in 111 2/3 innings before the All-Star Game.

Potential fits: Giants, Padres, Braves

Nicholas Castellanos, RF

Castellanos' first four months were a nightmare given his upcoming free agency. He hit just 11 home runs with a .790 OPS in 100 games with the Tigers, putting him on pace for his worst season since 2015. Since being traded to the Cubs on July 31, the 27-year-old has eight homers and a 1.100 OPS in 24 games, showing teams what he can do for a contender.

Potential fits: White Sox, Cubs, D-backs

Gerrit Cole, RHP

Not that the right-hander needed more help to establish himself as the crown jewel of this offseason's free-agent class, but Cole has been a man on a mission during the second half. He was 9-5 with a 3.09 ERA in 19 first-half starts, but since the All-Star break, Cole is 6-0 with a 2.45 ERA, boosting his K/BB ratio from 5.67 to 7.45.

Potential fits: Angels, Yankees, Dodgers

Josh Donaldson, 3B

Donaldson inked a one-year, $23 million deal with the Braves last offseason, hoping for a healthy and productive season that would reestablish himself as one of the best hitters in the game. Mission accomplished. His 32 home runs and .903 OPS have helped the Braves seize control of the National League East, and he's been in the zone since the All-Star break, hitting 14 homers with an impressive .280/.399/.599 slash line. Even at 33, Donaldson should command plenty of attention this offseason.

Potential fits: Braves, Cardinals, Angels

José Iglesias, SS

The Reds' shortstop was already having a bounce back season with a solid first half, but Iglesias has excelled since the All-Star break, slashing .329/.349/.479 with four homers, 16 RBIs. His .828 OPS in 40 games is over 100 points higher than his first-half mark (.705 in 80 games), giving the veteran good momentum as he heads into free agency for his age-30 season.

Potential fits: Brewers, Pirates and Reds.

Dallas Keuchel, LHP

The lefty was having a respectable season after signing with the Braves in June, but a dreadful outing on Aug. 8 against the Marlins (3 2/3 innings, eight earned runs) bloated his ERA to 4.83 in 10 starts. Since then, Keuchel has allowed one run over 19 innings in three starts, helping Atlanta maintain its NL East lead -- and putting him in position for a strong finish as he enters free agency for the second straight year.

Potential fits: Braves, Phillies, Padres

Wade Miley, LHP

The unheralded star of the Astros' rotation, Miley has been as steady as a pitcher can be this season. He ranks fourth in the American League in ERA, has allowed three or fewer earned runs in all but two of his starts, and after betting on himself with a one-year deal last winter, is in position to cash in with a multi-year contract this coming offseason.

Potential fits: Astros, Yankees, Cardinals

Marcell Ozuna, LF

After posting a monster 2017 season, Ozuna had an underwhelming 2018 in his first season with the Cardinals. Year 2 in St. Louis has been much better, as the outfielder has already hit 24 home runs (he had 23 last season) while posting an .865 OPS, a huge leap from his .758 mark a year ago. Ozuna turns 29 in November and should have plenty of suitors this offseason.

Potential fits: Cardinals, White Sox, Giants

Anthony Rendon, 3B

Considered by most to be the top free-agent position player available this winter, Rendon is having the type of platform season that players -– and their agents –- dream about. The Nationals' third baseman has 29 homers, 104 RBIs and a 1.022 OPS in 117 games, posting an absurd .367/.443/.620 line with nine homers and 42 RBIs since the break.

Potential fits: Nationals, Phillies, Cardinals

Zack Wheeler, RHP

The Mets right-hander had an up and down first half, but just as he did last year, he's made big strides since the All-Star break. Wheeler, who had a 4.69 ERA in 19 first-half starts, has a 3.72 mark in six post-All-Star outings, though his strikeouts per nine innings are down from 9.8 to 6.4. Still, he's considered by most to be in the second tier of free-agent starters and should land a nice deal.

Potential fits: Yankees, Angels, Red Sox