In reply to victim of mathematics:
> At the lower grades, the hardest part is often avoiding the urge to climb around the jamming.
^^ This is the thing.
For easier jamming, it's absolutely everywhere on the grit but most people who don't like jamming just don't notice it.
On Peak Rock says something about that - Heather Wall (Froggatt) is Jamming Test No 1 but as the book says the test isn't whether you *can* jam, it's whether you choose to.
Let me chuck in a few more suggestions to add to the ones already made though..
Embankment 2 at Millstone (*so* much quality jamming at Millstone).
Jam the wide crack and place gear in the narrow one. When/if you find you're really getting the thin finger jamming sorted, you can jam the narrow crack and place gear in the wide one.
Mutiny Crack at Burbage North is a must. (It's only HS, but a dreadful sandbag for non-jammers. The crux is right at the bottom.)
Ignore any suggestions to do 20 Foot crack at Burb N by the way, its shiny, slippery and horrible. Go and do the Real 20 Foot Crack at Stanage instead.
It might actually be better to learn the technique in more of a bouldering style: Dog Leg Crack at Curbar is perfect for that. It's (just about) a highball with perfect hand jams, the tricky bit is the change of angle brought about by the 'dogleg' itself. The Pugilist (just to the right of the Eliminates wall) is another great little struggle, but as the name suggests it's more about the fists. (And a bit of an offwidth-y start - it packs a *lot* of fun into about 4m of climbing!)
There's an amazing little 'splitter' hand/fist crack at Moorside Rocks, just to the left of the Jackalope. To make the most of it start sitting on the boulder at the foot of it. That is one to tape up for though - it's full of sharp crystals and even with perfect technique it'll try to eat you.