You didn't say what you want to use it for. RISA, like JAE typed, is a fantastic 3D matrix analysis program, although it mostly sticks with the basics: small displacements, no time history analysis, etc.
If you're interested in handling large displacements, cables, time history, predicting frequency response functions, power spectral density, eigenvalue buckling, virtual work optimization, staged construction, etc., then SAP is a great choice for the money. I view it as an "in betweener" because it does far more than the basic packages, but the next level up is ansys, abaqus, etc., which are FAR outside the need (and budget) of most engineering offices. SAP does most of the useful stuff that those do and costs 3-6x less. ETABS does a lot of this stuff also and is more specialized and friendly for bldgs.
Now excuse me while I step on my soapbox (stop reading here if ya don't wanna hear it, LOL). I'm personally a bigger fan of the "non-black-box" programs. In my old office (worked for 10 years and came back to school for my PhD) we used RISA and everybody still knew how calc wind loads, design lots of things by hand, etc.
We hired one 12 year SE guy who had been using an un-named automated system for years and he literally couldn't manually calc wind loads.
We also tried to use an automated composite beam design system (also un-named, LOL) for one job. One of our decent 20 year guys used it to design a floor and it's like his brain turned off and the design was totally illogical. Very deep beams framing into shallow ones, no consolidation of sizes, every beam with a different # of studs and a different reaction, etc. This plan was fairly regular.
One of my pals own a large steel detailing company, so he design connections and sees what folks come up with for members. He said that he's lost track of the braced frames which had HSS12x12 with hundreds of kips axial, but a W10x12 beam. That's because the engineer didn't know his black box didn't let the beams have any axial load (rigid diaphragm).
I personally worked on a botched job involving mis-use of automated system, resulting in failure. The engineer accidentally had the program set to use shored construction, so had W12x14 beams, 34' long, 7-8' trib. Girders were W18x35, 34' long. The first bay literally started to collapse when they put concrete on it. He obviously pressed the "Run" button and then never looked at it again. This was from a reputable firm and was supposedly one of their experienced folks. Obviously, the owner wanted to know what somebody besides him had to say about how to fix it, LOL.
Long story short: If you (or anybody else) implements a black box system, intensive quality control procedures need to be in place. If I were to allow use of one of these, I would mandate that every floor plan be plotted to 1/8"=1'-0" scale and EVERY beam be highlighted by the responsible engineer. In other words A HUMAN will *mentally register* what sizes are used everywhere.
Sorry for the soapbox. I believe there is are serious problems from mis-use of these programs.