Butler, 26, split the 2012-13 season between the Predators and New Jersey Devils, combining to total four goals and 11 points in 34 games. Butler previously played three seasons with the Ottawa Senators.
Brennan, 24, split last season between the Panthers and Buffalo Sabres, combining to total three goals and 10 points in 29 games. Brennan was a second-round pick by the Sabres in the 2007 NHL Draft.
Each player can become a restricted free agent this summer.
I don't know how he does it. I don't know how he gets his body parallel with the player and pulls it through his legs like that. I know he's tried it a couple times in practice and it's never worked, so how he does it in a game, it's incredible.
— Capitals defenseman Mike Green on teammate Alex Ovechkin's highlight-reel goal against the Devils on Saturday