Well, it's free [gratis], but it certainly isn't open source. The source to this version isn't available under any licence, let alone an OSI-approved one.
The original post here is a bit misleading. I downloaded the file shown at SourceForge, and all it seems to be, unless I missed something, is the executable. There is no source provided. There is a mention in the license that the source can be obtained from SourceForge, but perhaps you can give the link if you find it.
The license seems to be their own proprietary license, its first point is:
1. Ultra Enterprises ("UE") grants to you a non-exclusive, non-transferable license to use this version of the Pueblo/UE client ("Software") in executable form.
You can see the license is for the executable, not the source.
After a bit of browsing I found this paragraph on the supporting web page ...
Pueblo 2.02 Source
Note that the Pueblo/UE source hasn't been published yet, since after a year or so of tweaking it's really messy. I'll try to get that out as fast as possible (though if you absolutely positively can't wait, it's available on request; this is open source after all). In the meantime, here is a link to a page at MOO Canada where you can download the binary version of the original Pueblo 2.01 as well as the Pueblo 2.02 source code. Though be warned, the code is really old - hence all that tweaking!
So, to summarise, the source isn't published at all, and the only link to source is the original Pueblo, which I suppose is not "just as it was intended" but the original one which the original authors abandoned. I can't see where the open source license is, maybe it is in the source which you can get by applying to the author.
BTW - this isn't really the place to advertise competing clients. :)