As of 2008, Adaptive can be found on hackage.
However, this Haskell library provides a monadic interface, which doesn't need the "write" operation or the "destination" type. In addition, the monadic types enforce correct usage, which means that a modifiable variable must be defined before it can be used. This is achieved within Haskell's type system plus some popular extensions (multi-parameter classes and functional dependencies). All of this is described in the ICFP'02 paper Monads for Incremental Computing.
The library is parameterised over any monad that has references (such as IO and ST). This means that it should be possible to put it on top of e.g. many GUI monads too.
There is a small demo program of a classical incremental computation problem: a spreadsheet. Try it by typing
runhugs -98 spreadsheet.hs
and type, say
c0 <Return> Cell c1 <Return> c1 <Return> Const 42 <Return>
This has been tested with the Hugs February 2001 version.
Feedback is welcome!