Free Agency in Japan works differently than free agency in the Major Leagues. Players earn domestic free agency after eight seasons of service time in the NPB, meaning they can exercise the option (assuming they haven't signed a new deal) to become a free agent and sign with another NPB team. However, unless a team agrees to post a player, they have to wait until 9 years of service time to become (or have the option of being) an international free agent, meaning they can sign with teams overseas (read: the Major Leagues).
So I compiled a list of the players that have international free agency eligibility* and are still active. Some of the players are backups or are no longer very good, but that is okay for this list. I also put their NPB salaries converted to U.S. dollars (using 1 Yen=.010 dollars), because all other things equal, you would expect an American team to have to pay them more to get them from Japan. Of course, this isn't always the case, as some players may decide that no amount of money is worth leaving their home country and some players may decide that taking less money for a shot at making the Majors is worth it. It is just to be used as a measuring stick. I also put what the expected MLB WAR, using the 5 million dollars equals one WAR measuring stick used by FanGraphs and others to evaluate free agent contracts, next to the salary. In a world where the player would take the same amount or more money than his current NPB salary to play in the Majors, they would need to exceed that WAR to be "worth it".
As you may notice, NPB salaries are pretty low when compared to MLB free agent contracts, so nearly every player on the list just needs to play as an effective part time player to be worth their current salary in the Majors (obviously some of them are playing for less than 500,000 dollars, which is below MLB minimum). Below, I will take short looks (by no means comprehensive scouting reports) on three players that are at least interesting to me as international free agents. I have no idea whether or not any of them would consider coming to the States, so this is purely a talent, not a conjecture of their career desires, perspective.
Shinnosuke Abe has been perhaps the most prolific and well known hitters in Japan over the last several years. Despite turning 34 before the 2013 season started and really struggling to move in the WBC thanks to a previous knee injury, Abe still catches nearly every day and is enjoying a great offensive season. In fact, by OPS, he is enjoying the best season of his career, even better than his prime years before the first ball change (he hit 44 homers in 2010, the last year of the "old" ball). Only Wladimir Balentien (who is chasing the single season HR record in the NPB) and Tony Blanco have a higher OPS in the Central League (with a minimum of 50 games) than Abe, meaning he has been the best Japanese native in the league. Here is a GIF of an Abe homer to get an idea of his swing:
poor job MLB teams have done at evaluating NPB hitters, hesitance over guaranteeing 6 million dollars to a guy just for his bat would be understandable. With that said, his plate discipline looks like it has improved over the past couple of years, as he is now walking more than he strikes out, and by a considerable margin. An interested team wouldn't be just counting on his power to play, but he has shown the ability to hit for consistently high averages with good plate discipline, giving the power more of a chance to play up at a higher level.
As far as I know, Abe hasn't expressed interest at coming over to the Majors, but he is probably the best bet to be a successful hitter as a free agent (considering 2013 brought another disappointing class of middle infielders from the NPB, expectations and interests on Takashi Toritani is most likely low).
Two other somewhat interesting (but considerably less interesting) possible free agents in my opinion are pitchers. The first is a NPB starter, but someone who would pitch out of relief in the Majors.
Masanori Ishikawa is a 33 year old left-handed starter for the Yakult Swallows. His greatest claim to fame is most likely his ability to avoid walking hitters, with just a 4.2 % rate in his career, and 5.7 % rate over the last two years. As you would expect, that doesn't come with a lot of strikeouts either, with just 13.3 % for his career, and 13.9 % and 14.2 % over the last two seasons. He is also listed at 5-6 161, which despite a consistent run of 170-190 IP seasons, isn't going to interest MLB teams as a starter. However, he could be a lefty specialist, shown by his ability to limit lefty hitters to the Mendoza line this season. His release point also looks like one of a lefty specialist, as shown by the GIFs below, showing three of his pitches.
He also throws a notably slow curve and has been known to let go of an occasional screwball. He is a guy with a decent pedigree of success in the NPB, can throw strikes, and provide some deception. The big problem is that MLB teams will probably not want to pay 2 million dollars for a left-handed specialist that hasn't proven himself in the big leagues yet.
Shinichiro Koyama is a 35 year old right-handed reliever for the Rakuten Golden Eagles. Despite a decline in his peripherals in 2013, he has been able to keep his earned run average near where it was in 2012, where it dipped below two. There isn't really anything flashy about Koyama other than he has finished some games in the past and throws a pretty good changeup.
*Thanks to Jason Coskrey of the Japan Times for double checking my list, which mostly came from yakyubaka.com's research.