Synchronising multiple calendars with multiple devices

Hello everyone. 

I would really appreciate some advice from someone who knows more about all this than I do.

I work on an iMac and also have the other Apple products (Macbook Air, iPad 2 and iPhone 4s). 

Until recently I have been using iCal for my calendar and it synchronises well between these devices. The problem is that iCal invitations are not always recognised by other people's Outlook calendars. It is important to me that I am able to send calendar invitations so I am looking for an alternative calendar system. 

I am thinking of using Outlook for Mac 2011 because I can send calendar invitations from that system using my normal email address (a web based email account that contains the domain name of my business) which keeps things nice and simple for my clients. Previously I had sent calendar invitations  from iCal using gmail which also updated across devices  but was confusing for people because then they sent emails to my gmail account which I don't want them to do. Outlook allows me to send calendar invitations from my normal business email account.

I work for various companies and sometimes need to send calendar invitations from other email accounts too - the type of these vary: some Exchange/POP/IMAP. So I cannot just set up my own exchange system. I need something that will accommodate a variety of accounts and critically, update all my devices. It would be wonderful if it could provide an online platform to do all this as well - so that I can create and see all my appointments on all my devices and also online.

So my big question is this: is there a solution that will help. I'm guessing that there might be but I have no idea.