Can anyone recommend a simple tool for benchmarking a virtual server?

Everyone quotes equivalent CPU figures (eg one one server I have the equivalent of 1x0.35GHz Xeon cores) but I'd like a tool that would give me a quick benchmark that I can run on several virtual servers to make comparisons between them. I don't really care if or how the results compare with "real" CPU's, I just want to know if server A is better or worse than server B, and ideally by how much ("twice as good", etc).

I don't want to hammer the virtual server or run tests for hours or days.