>> If you're in ruby, I prefer this gem, which can check ISBN check digit as well as convert from 10 to 13.
>>
>> https://github.com/entangledstate/isbn

> Good to find another isbn validator.  I use: http://isbn-tools.rubyforge.org/

> Not sure why there's two nearly identical gems, exactly.

Probably cause whoever wrote the second one didn't realize the first one was there, it happens. 

These days all the ruby action is in github.  When presented with two gems doing much the same thing, I'll take the one that has the cleaner api and/or has been maintained more recently, figuring it's got a better chance of being updated to work with ruby 1.9 if needed or what have you.  If one of em is on github and one of em is on rubyforge... I'll take the one on github, because it makes it so much easier for me to look at the source code/README on the web to see which one's got the better api, and/or to look at when the last commit was (and to send pull requests if I find a bug I want to fix, or to fork if I find a bug and the maintainer ignores me). But that's just me.