http://www.ViaDesigner.com is free, easy-to-use schematic capture and simulation that you use in your browser. Public designs are free and you start paying if you want private, team or implementation features. ViaDesigner lets you combine analog, digital and mixed-signal circuits in a unified simulation environment that supports VHDL-AMS, SPICE, VHDL and Verilog design entry formats.
VHDL-AMS sounds pretty complicated but you don't need to learn how to write VHDL-AMS or SPICE models to use ViaDesigner. Instead, ViaDesigner comes with a set of high-level design wizards that generate useful mixed-signal circuits like: gain stages, filters, analog to digital converters, DAC, linear & switching regulators, etc. Example wizards can be found here: http://www.viadesigner.com/viawizards/.
Here's some screenshots of the ViaDesigner schematic & simulation environment:
Ultimately, ViaDesigner.com is focused on letting you design your own mixed-signal custom chips easily and cost-effectively. ViaDesigner will soon have a complete IC design flow in the cloud. This implementation design flow isn't free but the goal is to enable custom mixed-signal IC design, verification and fabrication for about $10,000. That's a lot of money but typical IC design flows can be anywhere from $100,000 to $1-million.
ViaDesigner is currently in an invitation-only beta. If you would like to receive a beta invitation then visit the sign-up page http://w2.viadesigner.com/viadesigner-beta-info-page
Yes, I work at ViaDesigner & these are my biased opinions about the ViaDesigner.com and the ViaDesigner software but still I'm pretty excited about what we are putting together.