Background and definitionsThe Insight section of the Proudman Oceanographic Laboratory site has a good overview.
The Tides questions Answered section had a very good detailed description of the factors influencing the tides.
"Our Restless Tides" , http://co-ops.nos.noaa.gov/restles1.html , gives a very detailed explanation of the factors behind tides and currents.
In the Ashore section, I link to a Coastal Navigation tutorial site. Section.6 of this opens with a succinct description of the factors infulencing tides - http://http://www.sailingissues.com/navcourse6.html
Easytide - from the UK Hydrographics Office
If they don't know, who does? This link is to the main Easytide site. The drill-down to a port is a three-step process, so once you are in, they have a useful option to bookmark your own ports. Ports are UK, Irish and worldwide.
They note that the times to NOT take Daylight Saving Time into account.
National Tidal & Sea Level Facility
Provided by the Proudman Oceanographic Laboratory, this also offers tides for 'Today' and 6 days ahead
The Weather section of Yachting and Boating World has a link to 14-day tides at UK and irish standard ports.
XTide: Harmonic tide clock and tide predictor
But first - a rant !
An interface to XTide with options:
See http://www.flaterco.com/xtide/ports.html , for information and for pointers to versions of XTide for Windows, Palm Pilot, etc.