As Ben noted, the Senators traded up-and-down offensive defenceman Joe Corvo and injury-plagued young winger Patrick Eaves to Carolina for defensive stalwart (but offensively challenged) Mike Commodore and injury-plagued winger Cory Stillman. Both teams seem to have assessed their current positions, and made the change to head in a new direction.
In Ottawa, it's been well-documented that the recent slide was a result of defensive gaffs, and unloading Corvo might cut down on those. By replacing him with the younger Commodore, there's another physical defenceman who can make opposing forwards pay and also plays a responsible style defensively. While there's a definite loss of offence from the defence--Corvo was a big part of the powerplay, not to mention his even-strength abilities--the addition of Stillman may offset that.
Stillman offers veteran presence and brings some offensive depth--which Bryan Murray has made no qualms about seeking. In playing beside Mike Fisher, Stillman will bring something to the second line that has been missing, and will make our second powerplay unit reasonably respectable. Not to mention the Stanley Cup rings both new guys bring, evidence of their abilities. Even though Eaves is a price to pay, his injury problems have made it difficult for him to keep any sustained offensive numbers. Also, with Chris Kelly and restricted free agents like Andrej Meszaros and Antoine Vermette, it would have been difficult to re-sign Eaves.
Even though both Stillman and Commodore are unrestricted free agents after the season, neither is making a lot of money this year, and it's possible we could get a bargain like that again.
For some reason, I don't think that Murray is done adding players to this team. I have a feeling another top-six forward could be on the way.