Yasm is very good. it uses NASM and GASM syntax, making it very easy to learn, it's cross-compatible with linux and windows environments and can compile assembly code for both x86 and x86_64 instruction sets.
the core focus of the project being on it's libraries, it's very easy to integrate cross-compatible assembly into C or C++ code and use your normal C++ IDE with minor modifications to support the NASM and GASM syntaxes.
and no, i didn't quote anything from the website.
(note, the "very easy" is qualitative, not a complete fact, since it is a project and not a completed product, the same challenges present in other project-oriented code are quite possible)