My personal favorite is Linear Technology's SwitcherCAD III. It's totally free, and not castrated as most evaluation or student versions are. You need some knowledge of Spice syntax (.lib, .op., .step, etc) to really be able to use it.
I also use SwitcherCAD III, it's got a really intuitive interface so it's easy enough to learn, completely 100% free, the parts library is updated ALL the time, and it's just an all around great simulator. I've used quite a few, and it's definatly my preferred choice.
I will third that notion about SWcadIII... especially good for starters and ya cant beat the price! However, like everything else in this world, it does have its limitations and a finite usefullness. If you plan on doing more advanced types of analyses (monte carlo, pole-zero etc..) then SWcadIII wont provide you a solution.
AS for a commonly used Pspice? Orcad student version or the multisim Pspice (student) is commonly used. But IMHO Swcad is better than these crippleware anyways.
In my mind, the jury is still out on what is all around best Price/performance ratio Pspice offering available today for less than a few hundred dollars (which is what I am willing to spend if justified) but I am
leaning towards one called VisualSpice. For $150 USD the capabilities you get is phenominal (in the advanced/expert version) but we'll see..