Changelog

The default value for the hash argument of memoise() is now taken with an indirection in case memoise() is called at the top-level of a namespace (r-lib/rlang#1177).
Fixed a bug in has_cache() that caused it to get the value unnecessarily. (#123)

Memoise now uses caching objects from the cachem package by default. These caches support automatic pruning, so that they won’t grow indefinitely. The older-style cache objects in the memoise package are still supported, but we suggest using new-style caches from cachem. (#115)
Name clashes between function arguments and variables defined when memoising no longer occur (@egnha, #43).
Add Google Cloud Storage support via cache_gcs() (@MarkEdmondson1234, #59)
Add compress option for non-memory caches (@coolbutuseless, #71).
Use absolute path in cache file system backend, so user can change working directory after using relative path (@xhdong-umd, #51, #65)
Add drop_cache() to drop the cached result for particular arguments (@richardkunze, #78)
Suppress messages of aws.s3::head_object within cache_s3’s cache_has_key to avoid printing of 404 messages for new keys (@stelsemeyer, #96).

Caches now hash the function body along with the arguments, to ensure functions with identical arguments use a separate file-system cache. (#38)
Handle missing arguments in memoised functions for simple cases not using non-standard-evaluation (#19).
memoise() gains a cache= argument to specify an external cache. Two types of caches are available, cache_s3() for amazon S3 and cache_filesystem() for a file system cache (#25, @danielecook).

memoise() now signals an error if an already memoised function is used as input (#4, @richierocks).
has_cache() function added which returns a boolean depending on if the given call is cached or not (#10, @dkesh).
Memoised functions now have a print method which displays the original function definition, rather than the memoisation code (#15, @jimhester).
A memoised function now has the same interface as the original function, if the original function is known when memoise is called. (Otherwise, the old behavior is invoked, with a warning.) (#14, @krlmlr)
The enclosing environment of the memoised function is specified explicitly, defaults to parent.frame().
is.memoised now checks if the argument is a function.
Testing infrastructure, full test coverage.

New features

Memoised functions now have an attribute memoised=TRUE, and is.memoised() tests whether a function is memoised. (Contributed by Sietse Brouwer.)

Documentation is now more elaborate, and hopefully more accessible to newcomers. Thanks to Sietse Brouwer for the verbosity.