Race Face Charge Knee - would love to tell you about them from personal experience but the chunts at UByk still haven't sent them to me 4 months after placing my order. They are a knee-warmer with a protective pad on front that prevents only abrasions / cuts .. which may be all you're after - what's the doc say?
Fox Launch Enduro - someone took an old diver's wetsuit, cut out 20cm from the knees, sewed some extra rubber on the front and a little mesh window on the back. The breather holes, they do nothing! They weren't terrible, per se - I just didn't like riding in wetsuit knees. Better for mid-winter somewhere well into the lower half of Aus maybe? I liked indoor climbing wearing one, after I had some stitches out of my knee..
TLD Speed Knee - light but more serious pad than the RF Charge. Thick knit with maybe 4mm of D3O padding. After several hours on a hot day I think they might have been rubbing away a bit of skin on the back where they crease as I pedalled. Worn numerous other rides, they're nice enough.
TLD Raid - tried these on in an LBS, nothing more. The Speed Knee M/L is the right size of me, but I seemed to be between the XS/S and the M/L in the Raid. Seems like a pad that offers great (AM/Enduro level) protection and comfort.
NukeProof Critical Enduro Knee Sleeve - bought them, tried them on, they seem like might be a nice pad after breaking them in a bit. Too many other options now ..
POC VPD Air - I don't particularly like the whole "We'll charge a million dollars for all our stuff cause we're swedish and safety like volvo" thing but these are a nice sized and shaped blob of D3O-alike "Viscous Polymer Dough" offering decent protection against impact. And I raced 100km XC in them without discomfort. There's the VPD 2.0 and 2.0 DH versions for going harder, too.
~2004 Fox 911 Knee/Shins - They offer hardshell protection to knee and shin front. They flex effortlessly where the pad narrows between the knee cap and the shin guard, and sit in place OK with just 3 relatively narrow straps. They don't weigh much and dont strap over the back of your knees. The shells could have been a bit thicker, but otherwise perhaps a better trail/AM/whatever level pad than so many others on sale today?