Everyone has mentioned Shimano SPDs, which I have used previously, but I have recently switched to Speedplay Frogs, and believe they may be the solution for many people. I'll try and explain my reasons...
I loved the definite click of the SPDs, and found releasing was fine as long as I you remembered to do so! However, whilst cycling End 2 End last year, my knees gave my serious problems. It may have been the exertion of the trip, but I believe the pedals were something to do with it.
Shimano SPDs and, I believe, most other clipless pedals, involve some degree of "self centring". In other words, although there is some degree of "float" (where the foot can swivel 15 or 20 degrees without releasing), the spring mechanism is always gently pulling the foot toward the "centred" position.
This means that when setting up your SPDs, you need to ensure the cleat is fitted to your shoe at the correct angle. The correct angle is one which means when the SPD spring "centres" the shoe, the foot is being held at a natural angle. The problem is that if the cleat is marginally "out", when cycling your foot is gently pulled into a non-natural position.
For many people this is fine - they can get the cleat spot on, or they have no ill effects caused by a wrongly set up cleat. I believe though that anyone with knee problems needs to be exceptionally careful.
So, on to the Speedplay Frogs...
I'll let you visit the Speedplay website to look at photos and to see how the pedals work. However, I would say that these pedals are great for the following reasons:
1. There is no self centring. You still have "float", but there is no spring to pull the foot inwards.
2. The foot moves very freely - there is no resistance to pivoting the foot whatsoever. I would liken it to having your shoe on an ice cube - it is that smooth!
3. The mechanism is very simple and there is no way it can get clogged with mud. This is very different to SPDs where dried mud can prevent entry and removal.
4. There is no resistance to removing your feet - at first you will find you remove your foot without meaning to!
A few words of caution:
1. The Speedplay Frogs feel very different to SPDs. As I said, at first you will find you remove your foot without meaning to. But stick with it - you'll soon get used to the change.
2. There is no definite click when you slot your foot in - again this soon feels "right" - stick with it!
To finish...
I bought some Speedplay Frogs and some XTR SPDs a couple of weeks ago. My plan was to send back whichever I didn't use. After 3 miles I HATED the Speedplay Frogs. I was furious and close to sending them back. I stuck with it, and after only another 3 10k rides I am converted. I don't know yet how my knees will be after I go cycling in France this summer, so I will let you know.
I hope that is of some use.
Neil
ps I don't work for Speedplay, I just really like them!