NPB teams typically use interleague play as an opportunity to bust out the alternative uniforms. Interleague started earlier this week, and this season is no exception.
The SoftBank Hawks introduced their third uniforms yesterday, and Sanspo has pics here and here. I think I prefer this to their normal third jersey, but I hope at some point they bring back the helmet the old Daiei Hawks wore in the late 80's.
The only other team to have introduced their throwbacks so far this year is the Hanshin Tigers, who have sensibly opted to revive their 1985 Nippon-ichi uniforms. If you've been following Japanese baseball over the last few years, you are probably aware that Hanshin has a checked past when it comes to alternate jerseys: last year's awful fade/airbrush design, 2007's alternate logo and yellow pinstripes on black, and the earlier 70's era throwabacks, which I kind of like.
Seibu has plans to wear 80's-era throwbacks, and Yakult is going to dust off their old Kokutetsu Swallows design, but neither team has introduced the uniforms yet, so don't bother with the links unless you're interested in re-reading what I just wrote in Japanese.
My favorite recent throwback was last year's Lotte Orions revival. I hope they use those again. Lotte has one of the best home uniforms in Japan, the classic black pinstripes on white. But their road and alt uniforms... not so much.
For more on Japanese baseball uniforms, check out the two posts that UniWatch ran last month. UniWatch produces so much content it's almost a sensory overload, but they don't miss a thing and it's worth the read, especially when you can find something like this set of 1981 Chicago White Sox prototype uniforms.