As noted by MrSouthpaw, the Pirates have settled prior to arbitration with lefty reliever Antonio Bastardo. Jon Heyman has reported that the amount is $3.1M, a little above his MLBTR projection of $2.8M. Bastardo made $2M with the Phillies last year. This is his third year of arbitration eligibility; he'll be eligible for free agency after this season. The Pirates acquired him for LHP prospect Joely Rodriguez.
I'll update this post with other signings as they come in. Despite their seeming brinksmanship, the Pirates seldom go to a hearing in arbitration cases.
UPDATE: Ken Rosenthal tweets that the Pirates have settled with C Francisco Cervelli for $987,500, a little below the MLBTR projection of $1.1M. He made $700,000 with the Yankees last year. This is his second year of eligibility. The Pirates acquired Cervelli for LHP Justin Wilson.
UPDATE: Josh Harrison parlayed a big 2014 season into a $2.8M deal in his first year of eligibility. HT to Thunder.
UPDATE (by Vlad): Per Bill Brink, Jared Hughes has also settled prior to his hearing, for $1.075M. HT to Thunder, again. MLBTR's prediction for him was $1.1M, so it was also fairly accurate.
UPDATE: Brink tweets that the Pirates have settled with closer Mark Melancon for $5.4M. The MLBTR estimate was a quirky $7.6M that probably resulted from their formula counting him as both a closer and setup man.
UPDATE: Mike Perchick tweets that the Pirates have settled with Tony Watson at $1.75M and Travis Snider at $2.1M. MLBTR also links to a Perchick tweet saying Sean Rodriguez has signed for $1.9M, although I can't find that tweet in Perchick's current timeline. If these are all accurate, that would leave Neil Walker, Pedro Alvarez and Vance Worley as the only outstanding, arbitration-eligible players. Rob Biertempfel has stated that Walker did not settle and will head to a hearing.