Google Calendar can send reminders to guests, if they have a Google Calendar account and they choose to add a reminder.
Otherwise, specialized event management apps include:
- Meetup.com: sends reminders
- Smart Events (free for less than 5 attendees)
- EventBrite (free): sends reminders
- Eventzilla (free): sends reminders
- TicketLeap (free): sends reminders
- Epic Event (free if the event is free): sends reminders
- Eventful (free): sends reminders
- DoAttend (freemium): sends reminders
- Ticket Tailor (freemium)
- Amiando
- TicketForEvent (freemium)
- BookWhen (free)
- Guestlist (freemium)
- MyGuestlist: sends reminders
- SimpleTix: sends reminders
- Stage (freemium)
- TicketPeak
- EventArc (freemium)
- EventStorm (freemium): sends reminders
- TicketDerby
- RegOnline (free): sends reminders
- Ticketbud: sends reminders
- CharityHappenings Ticketing (free): sends reminders
- EventElephant (freemium): sends reminders
Most specialized event management apps can send automatic reminders to users.
I've indicated the ones that I know can send them, either from experience or because it's stated on their site on in reviews. Italicized entries mean I don't know/couldn't confirm that reminders are sent.
But you can usually find out by googling *service name* reminders or *service name* notifications, visiting their site, or contacting their sales/support.
Other options include Yahoo! Events, Live Events, Facebook, and Myspace.