There are many ways to cross The Channel ('La Manche' in French): which method you choose will depend on where you live, when you want to cross, how much riding you want to do on either side of the Channel, the cost and, in summer, quite possibly what hasn't already been booked.
There are many ports on this side served by a number of ferry companies: click on the company titles listed below to take you their respective web sites for more detailed information. The nearest ports to us are St.Malo (about 45 minutes away), Caen (90 minutes), Cherbourg or Le Havre (each around 2 hours away).
Click on the bold underlined names for further (off site) information...

Condor Ferries: offers Poole or Weymouth to Cherbourg or St.Malo via the Channel Islands including a Fast Cat service. Reasonable service, reasonable costs although their timetable can be a bit bizarre. The Fast Cat is very quick, but has had reliablity problems in the past.
Brittany Ferries: has numerous crossings from Portsmouth to St.Malo, Cherbourg and Caen including some fast crossings (although be aware that, if you're travelling en masse, the FastCat boats only have limited space for bikes - usually a maximum of twelve). In addition, if you live in the West Country, consider the Plymouth - Roscoff route. Roscoff is on the North West coast of Brittany, but a very easy pleasant three hour ride away. This long established ferry company offers excellent service and a very good choice of sailings on good stable boats.
LD lines: LD lines up until recently had only one boat on one route which meant just one ferry crossing a day (Portsmouth to Le Havre) with a sailing duration from 5 hours - a second boat (The Norman Voyager) started running on the same route from November '08.
(N.B. Back in December '08, I posted some misgivings about the lack of proper thought and equipment given to securing bikes on this new boat, but following some correspondence with LD Lines, I received this reply:
"I have brought your query to our operations Manager who has confirmed to me lashes and cushions will soon be available and some fastening points will be installed"
May 2009: I've just crossed the Channel on the new Norman Voyager and I'm happy to report that the crew lashed the bike securely using brand new equipment. It's fair to say that LD Lines do have a reputation for listening to their customers and this seems to bear that sentiment out. Their fares are also the cheapest across the Western Channel)
In early 2009, they also added a boat for the Dover to Boulogne route and a Dover to Dieppe route: both relatively inexpensive ways to cross the Channel.
Transmanche Ferries: ply the Newhaven - Dieppe route. They offer four ferry crossings a day with a sailing duration from around 3 hours. I used this company for the first time in November 2007 and was impressed: the boat is well appointed and the fare was very reasonable.
P&O Ferries
offers around up to 30 sailings daily between Dover and Calais or Boulogne. The crossing duration is from 55 Minutes. Very convenient, but once again, a bit of a drive on the French side before you'll arrive here.
Sea France: As of 9th.January 2012, Sea France has ceased trading...
If you're considering the Dover route, there is always the:
Euro Tunnel: a very quick and convenient method to get across, or rather under, The Channel. By the way, I came across this piece of advice on a forum recently:
"If you book online, where it says "Type of privilege" choose "Friend of Staff" and in the box that says: "Membership N° or reference" type: "004732shut". This gets you 40% off, at least it did last time I tried it"
I haven't actually tried this, so don't blame me if it doesn't work...