Eventful and Upcoming are two very similar "event database" systems, which host event listings, allow you to add or search for events, and allow you to filter them by various criteria or tags.
I think Upcoming is pretty good, but Eventful wins in my book for one simple reason: I can click on a button in an event, or list of events, and open it in my calendar program, currently Outlook, as long as that program handles files of MIME type "text/calendar". This makes it really easy to move an event listing to my own personal calendar, which is where it needs to end up. I've only tested this on my PC - it might work on a mobile phone, if that phone's software can handle opening the file type. I'm told the PalmOS can open it, but that may only be in e-mail, and not a browser. Even so, it's very useful.