As I think that this website should not be tied in to just one simulator, this page contains several circuits for simulation on the Micro-cap simulator.
The categories covered include, some really simple circuits, simple filters, the common emmitter amplifier, and op amps and comparators.
This section contains several simple circuits for simulation with Micro-cap.
Firstly download and install Micro-cap by putting "Micro-cap simulator" into a search engine
As most browsers just recognise the Micro-cap ".cir" files as text I suggest that you either....
download the ZIP files at the end of each group of circuits,
or right click on the individual circuits and use "save as" and then open the saved file with Micro-cap.