I've been using ZYB for a couple of days now. The reason is that since I lost my Palm Treo 650 I've had to use a Sony Ericsson i810w as a replacement which means I can't import my old contacts from Palm Desktop.
ZYB does two things: sync your contacts and sync your calendar between the zyb web app and your phone.
They support loads of different/popular phones (sadly not the Palm Treo) and if your phone is supported you download a little Java app that deals with the synchronization. Now I can add/edit/remove contacts and calendar events using the ZYB web app which is much more convenient than the tiny screen tiny keyboard phone. The worst thing about ZYB is the name. It took me a lot of time to first find the website since I had only heard of it and seen it on a distance. Since I wasn't able to remember exactly what it it was called (ybz? yzb? wyb? zwy?) I had to google search for it but just couldn't find it anywhere.
It's also got some wonderful extra features for exporting contacts. This is brilliant if you want a file backup of your contacts from your phone if you don't already have that. I'm currently looking into importing my old Palm Desktop CVS file of contacts but apparently the only way is going to be via using something called "Outlook" which I hear is a window$ program.