Can I use after the sell-by date?
Don't toss it. It's a "sell by" date, meaning it is still safe to be sold on that date. Not an expiration date, or a ticking time bomb.
As long as you've used common sense with it (i.e. not lest it in the car on an 80 degree day for 2 hours, etc), then use or freeze is today or tomorrow. :)
I certainly wouldn't toss it...if it has been kept cold, it's still good beyond the "sell by" date.
You do want to use it as soon as you can...but as tzurriz stated, it's not a time bomb and doesn't undergo some kind of magical transformation on that printed date.
Chuck it if you feel nervous about it...but frankly, you'd probably be wasting a perfectly good piece of meat.