First, if you're in Denver, you don't have to worry about Southwest no working for you due to expiration, since Southwest doesn't come any closer than ABQ or OMA or SLC!

Second, I don't know of anyone else who does it "a la" Southwest. Southwest's program is completely different. With Southwest, your miles don't expire because you never earn any miles. You earn credits, and the same amount of credits whether you fly shorthops like LAX-LAS (which are often $75ish total round trip including all taxes and fees) or cross-country flights. Perhaps more importantly, the minimum amount they bookkeep is 0.5 credits, and you only need 16.0 credits (within 12 months) for an (automatic) free flight award (which you then have one year to book and fly). Thus the minimum they can give you for any activity is 1/32 of a free flight. (And, btw, that's a 50k award type of "unrestricted", walkup-equivalent flight except for a dozenish blackout dates a year.) So you know those rental car companies that give you a measly 50 miles per day? If they partner with Southwest (Hertz, Budget, Dollar, and Alamo do), they give at least 1/32 of a free flight per rental! You know those lower midscale hotel chains that only give 100 miles or so per stay? If they partner with Southwest (Hampton Inn's HHonors and all Marriott's except Fairfield Inn do) they give at least 1/32 of a free flight per stay! So if you do a number of rentals and/or stays a year at places that give you hardly any miles for that, Southwest may often be the ONLY thing worth earning toward (if you can avoid the expiration, of course).

Now, that certainly doesn't mean it's for everybody, but my point is that it has some major advantages that for a number of people (but far from all) compensate for the 12-month expiration. (Certainly, if I couldn't earn Southwest credtis so fast from partners, I don't know if I'd keep participating in their program, now that it takes 5.3 flights a year to keep from expiring by flying alone.)

So as you can see there can be no one "like" Southwest unless they also avoided miles for some bigger-granularity unit of measurement "a la" credits.