I am 99% sure Solarwinds releases no open source projects. the may release their server for free (as in beer) download but it is not open source.
http://www.gnu.org/philosophy/free-sw.html (Yes, I know there is a difference between Free and Open Source, but this is a link I needed an excuse to spam).
http://tftpd32.jounin.net/tftpd32_download.html
Is a free (as in speech) TFTP server (as well as half a dozen other functions). It is licensed under the EUPL which is an actual free licence, source code is on the download page now.
I might also add that I personally find it much easier to use (but that is anecdotal at best).