As others have said, health insurance is not a legal requirement for travelling abroad but it's best to have it.
It's also been pointed out that you should take an E111 form if you're travelling within the EU. (NB: E111's cease to be valid at the end of the year. From 2006 onwards you'll need a European Health Insurance Card).
Many people assume that, because they've purchased health insurance, they no longer need to take an E111 (or the new EHIC). This is incorrect. Imagine that you, say, break your ankle while abroad. The hospital will put your ankle in plaster and give you the X-rays to take home to your doctor. They will then present you with a bill. If you've got an E111 (or the EHIC) you simply hand them the relevant document and your troubles are over. If you haven't got an E111/EHIC, however, you'll have to pay a few hundred quid, there and then, and try to claim it back from your insurance company later. Now, if you don't happen to have a few hundred of quid on you . . .You've got problems!
So, within the EU, always take an E111/EHIC and CARRY IT WITH YOU!
Information about the E111 and the EHIC is here:
http://www.dh.gov.uk/PolicyAndGuidance/HealthAdviceForTr avellers/fs/en
Just a note on insurance: It's nearly always cheaper to seek out insurance independently than buying it from the company you booked your holiday with but remember to check the conditions carefully! (Most people know that winter sports - e.g. skiing - require special cover but many people have been caught out by not realising that watersports are excluded from their insurance. (These could include scuba diving, water-skiing, parascending, windsurfing or simply sitting on one of those giant bananas towed behind a speed-boat).
Chris