We have replaced the 4.6.17 release with a 4.6.18 release. This addresses some 4.6.17 regressions that notably affect CiviVolunteer and the estimating groups count on the mailing screen. There was also a pdf creation related error. The issues are to do with 4.6.17 security backports from 4.7 that missed something out.
If you have not yet gotten the security fixes in 4.6.17 you should choose 4.6.18.
If you have upgraded to 4.6.17 and are not experiencing problems you do not need to put on 4.6.18 immediately. We do suggest, however, that if you are not updating regularly (ie. not planning to put the new tarballs on more-or-less when they come out each month) you do update now, since you would not want to be using a release with known issues for a long period of time.
CiviVolunteer users should also update CiviVolunteer.