Recs.
Updated
Pharo is an innovative, open, integrated and highly productive eco-system comprising language (forked from Squeak Smalltalk), modern libraries (extensive), very rich and integrated IDE, meta programming tools, cross platform virtual machine(s) and a small but growing and thriving community including academics and industry partners. Now at Release 5 (stable, March 2016) with Release 6 well under way in development community.
Pharo can be used for server and client programming. It is open with easily used networking, parsing, generating and persistence frameworks.
SpecsUpdate
Pros
Pro Graphics, graphing and visualisation framework - Roassal
Roassal and Mondrian provide fantastic and easily used frameworks for graphics, graphing and advanced visualisations (comparable to D3.js) but with much less code. Visualisations can be rendered into web friendly graphics (SVG, .png etc.) without additional work.
Pro Glamorous toolkit & GTInspector
Most languages are still copying the Smalltalk tools of yesterday - GTInspector (written in Glamorous) takes live exploration of code/running objects to a new level. It's really slick, and better yet, you can easily write your own inspectors in 10 lines of code.
Pro Beautiful coding patterns in your IDE
No need to search google for compact beautiful examples of how to do things, your live environment source is available and you can easily live search, see how it works and copy how the masters would do it (examples most languages still copy too).
Cons
Con Small community
But they are very friendly and supportive. Best help comes through the mailing lists so not always easily googlable. There is also a Slack community where help is nearly instantaneous.
Con Suffering some growing pains as it embraces git
If you want extreme stability - you will have to look at older versions of Pharo as v6 & v7(beta) are embracing git (ultimately it feels like a good thing but it has meant you need to frequently update and cope with old and new ways of doing things).