[EDIT] The last version of MAMP with the last version of OSX Yosemite works fine.
I installed the beta of Yosemite two month ago, no problem with MAMP. With the last
I have had the same issue and none of the above worked.
The Apache server in MAMP wouldn't work on port 80, or 8080 (The default Apache ports). The issue is with Apache being enabled by default in Yosemite.
Either: change the Apache port in MAMP (Not preferable if you want to use the default port), or unload the launch Daemon by typing the following into the terminal:
sudo launchctl unload -w /System/Library/LaunchDaemons/org.apache.httpd.plist