There's been a lively discussion of the merits of the Haydn Sonatas on another thread. My question is this: Aside from Mozart and Haydn (and lets just agree that Haydn was great, if perhaps not as great as Mozart), was there anyone else working before Beethoven and the big boys of the early 19th century who did anything worth while? Clementi? Assorted JS Bach offspring?

Scarlatti! Do what I do (assuming you have/can find the editions) and crack open any sonata volume at random and just start sight-reading whatever sonata you hit. If you're like me, do that often and you'll find a great one to learn quickly. Scarlatti's influence on Mozart and Co. are pretty obvious when you listen to them closely. He was technically a Baroque composer, but he has plenty of Classical elements in his stuff. (Also, he's great if you love leaps.)

Clementi is alright, if not a little dry. I consider Scarlatti much more playful and creative.

Padre Antonio Soler and Friedrich Wilhelm Rust come to mind.

Steven

Certainly Carl Philipp Emanuel Bach. His sonatas and other keyboard pieces published in several volumes titled "für Kenner und Liebhaber" are especially lovely and quite adventurous in terms of harmony and form. Beethoven very much admired C.P.E. Bach's keyboard works and tried to convince one of his publishers to publish them (unsuccessfully).

And, of course, his baby brother, Johann Christian Bach.

Probably hundreds of composers who wrote for piano in the period you name. Whether they're "worthwhile" is a completely a matter of opinion.

You'll have to check out Johann Gottfried Eckhart and Johann Schobart.

Yes to Scarlatti of course - I've done exactly what you described Mr. Roberts Z. There's a terrific recording of these pieces by Schiff that I quite like. Agreed as well that Clementi is on the dull side. I played some of his Sonatinas when I was a kid - they were fun - but the Sonatas themselves seem kind of mannered and stiff. Kind of the worst of both the Baroque and the Classical eras. Of course I'm writing this in the hopes that someone will come back with an impassioned contrary opinion!

I'm really interested in the obscure stuff: had not heard of either Soler or Rust.

Pietro Domenico Paradisi, a contemporary of Bach, wrote a set of sonatas for the harpsichord. I recently listened to a lovely Toccata of his but I am not familiar with the larger sonatas.
Of course Arcangelo Corelli was a very prominent and influential composer but he pre-dated JS Bach with some overlap. His most recognized works are for the violin, but many are transcribed.

Most of the early 18th century composers who have remained famous composed in other genres. Important symphonists and opera composers like Galuppi, Sammartini, and Gluck.

There were a few who worked at the keyboard, though. Composers in the "Galant" style fit the bill - Alberti, JC Bach, and CPE Bach.

Towers the latter part of the symphony, Dussek and Clementi come to mind. Dussek's excellent works tend to be unfairly overshadowed by those of Beethoven, and Clementi tends to be dismissed as the sonatina guy. Ian Hobson also recorded some sonatas by Johann Nepomuk Hummel, but I haven't heard them.

some Clementi sonatas are quite good, and definitely no less than Haydn's. some pianists play some Clementi sonata in competitions as well. i remember one or two pianists from the 2005 Cliburn competition played Clementi.

CPE was not a Galant composer, he abjured the stuff especially his little half-brother's. JCF was. In fact, he took his 18 year old son to london to study with his younger brother. The son stayed for 5 years, even after JC's death.