iCal is adequate but its advantage with CalDAV (a protocol for communicating over networks/Internet) often was overshadowed by its lack of features, compared to Up2Date. Then I stumbled upon BusyCal which came with a 30-day trial.
BusyCal resembles iCal (Snow Leopard and earlier) yet what sold me was its ability to paste icons on to dates, weather forecasts and moon phases can be shown (up to five days and a month respectively), and notes. Plus it uses CalDAV or other cloud-based services if you wish.
The 30-day trial let me kick the tires for several weeks. Once I was convinced, Somara and I agreed to buy a family license (we saved $20). I’m still working out the kinks to get it to process invitations to people who do not belong to my server’s directory. This appears to be a gray area on what isn’t going through. However, if you receive something a Gmail address with this site as its name, fear not, it isn’t spam, it’s my calendar server inviting you to something one of us is coordinating.
Check it out here or the Mac App Store. The publishers have screenshots and it works with Lion.