C++ Super Simple HTML Tag Generating Code v0.1
At least in my opinion, there's quite a bit of room for improvement here. As far as usability goes, probably the most obvious change would be to directly support nesting. HTML and XML are frequently nested fairly deeply, but this doesn't really do anything to support nesting at all.

Looking at the code itself, I'd consider at least a couple of points:

1) You typically want to pass strings by const reference rather than value to avoid copying the string (unless you really need a copy, but you don't here). So:

std::string makeHtmlTags(std::string tagname, std::string tagcontent)


would change to:

std::string makeHtmlTags(std::string const &amp;tagname, std::string const &amp;tagcontent)


I would probably write ChangeHtmlTag to return the new string instead of modifying the existing one:

std::string changeHtmlTag(std::string const &amp;tag, std::string const &amp;newtagname)
{
    if(tag.at(2) != '/')
        return "&lt;" + newtagname + "&gt;";
    return "&lt;/" + newtagname + "&gt;";
}


Rather than using .at, I'd also consider just returning the input intact if the input is too short to be a tag, then do the modification if it's long enough to be a tag.

The biggest problem I see, however, is that the code just doesn't seem like it does enough/has enough intelligence to make it obvious that using it gains much over playing with the strings directly.