Ecentral.jp and Gaijinpot.com use similar formats for posting jobs, but their content is a little different.
Gaijinpot.com seems to focus more on language teaching roles.
Ecentral.jp (there is an English version just click at the top of the page to English or 英語) on the other hand, seems to list jobs for non-language teaching roles.
There are a bunch of roles on those sites and are kept up to date with new roles listed each week. However, you will definitely run into issues with language ability, work location and visa status which bar you from applying for certain roles. Also, many recruiting sites post generic info on both sites which can add a whole extra step to your job search if you choose to answer their ad. Applications to roles listed by actual employers are read by their HR, not by site staff. You can tell recruiter listed roles by their vague role titles and descriptions, plus "job serial numbers".
That doesn't mean you don't want to contact those companies too, as they probably have a lot of good info and you should get as much info as possible rather than wait at home hoping your dream role will magically appear by refreshing gaijinpot/ecentral's page every 5 minutes.