LTSpice is a free circuit simulator made by Linear Technology, as a result it contains many of their own components but it's still perfectly usable as a generic circuit simulator, a variety of common component models are included and you can add your own quite easily.
The interface is fairly decent although it could be a bit more user friendly, the simulation results are displayed in another window which may or may not be desirable, one of the best features is that you can pick voltage, current and power waveforms from the schematic and see the results instantly, the simulation is also quite fast compared with some other circuit simulators
The standard simulations are supported, DC operating point, DC sweep, AC, transient, noise, DC transfer function and parameter sweep (through the use of the .step command), SPICE commands and equations can also be used, the simulated waveform can also be analysed with a FFT.
The LTSpice simulator has developed quite a following (myself included) so there is a lot of help available if you run in to any problems, there are also various custom models and sub-circuits written specifically for LTSpice.
Overall LTSpice is an excellent and reliable simulator once you get used to the less than ideal interface, it's also fairly small at around 15 MB so you can easily keep it on a flash drive, currently it only works on Windows and Mac OS X but it runs fine under Linux using WINE.Download LTSpice