I don’t want the GNU, and would like corporations to use the software, but if they modify the original software, I want the license to state that this must be posted back as well or put up on a website or something.
What open source license would meet that requirement?
Take a look at the Mozilla and Eclipse Public License, or the LGPL (which is a little more aimed a C type early binding code…). I think both MPL and EPL will allow people to use your code as a library, but require any changes to your code to be made open source. IANAL.