Squeak provides some unique features among programming tools. It has ubiquitous debugging, non-stop execution even as the code changes, and snapshots. This combination makes for a very productive and pleasant working environment. I ofter wonder why it has not caught on more widely, and my best guess is that the original Smalltalk team thought of themselves differently than do language designers today. Language designers today write a compiler and then think of other tools as secondary. The Smalltalk team wrote what we would now call an operating system and its shell.
Over the years I have spent a lot of time on Squeak. My most notable coding contributions are:
Chuck: an 8-year Ph.D. project to bring type inference to Squeak
Islands: I spent another summer writing a secure-execution environment that is hosted within Squeak
Package Universes: a decentralized package-sharing system with dependencies and support for distributions
various contributions to the Unix port of Squeak, including three sound drivers (two for OSS, one for NAS) as well as being the central patch maintainer during long periods of non-official maintenance
MuSwiki: a swiki-like system where people exchange morphs instead of HTML
