If you are developing a free software (open source) project, you might be able to request access to the GCC compile farm, which appears to include at least one Mac Mini.
If you're not developing free software, it might be harder to find people willing to donate their CPU and bandwidth to you for free, especially since Mac hardware tends to be a bit expensive and Mac OS X is difficult to virtualize unless you pay half a grand for Mac OS X Server. There seem to be a variety of places which offer colo or dedicated hosting for Mac Minis and XServes, but not a lot of people providing free or cheap shell accounts on shared servers.