Ask HN: Best calendar solution for small online consultancy?

Calendar-ing is generally awful. What is the least worst calendar solution for small consultancies? All I want is a simple calendar that can integrate with Outlook if my client wants me to, but can also email invites to any email address and have that email be accepted by whatever software the client happens to use, Outlook or not. The answer may indeed be Outlook but, as I don't currently rely on any M$ solution, I thought I'd ask before unleashing the horrors of M$ dependency on my business.