Whatever you do, don't give up. Work with your vet to figure this out, and try the suggestions you find at this site.
My cat Fluff became a seriously aggravating inappropriate urinator. Anything on the floor he felt shouldn't be there, he peed on. On the plus side, this kept me from leaving clothes laying about the floor, but on the negative side if we didn't keep closets shut we'd find puddles in shoes, my brother rugby gear would be a whole new kind of stinky, and I went through 2 laundry bags.
This went on for a couple of years, and then he decided peeing on the floor beside his litter box was close enough. My mum was ready to get rid of him - but thank god he was MY cat, and while I couldn't bring him to me at the time, I could convince her to wait just a little bit.
Things we did: changed litter to clumping litter (we had been using Yesterday's News for it's dust freeness - but some cats will decide they don't like it.) Used Feliway on really important areas (ie around beds, christmas tree...it was expensive, even at cost, so used it sparingly) and stocked up on cat urine cleaning products to ensure we got the smell out when he "missed" the litter box. Finally, we put him on clomipramine during stressful times (eg. when I left to go back to school) as though times seemed to be the worst for him.
I finally got a house here in Guelph where I could bring him and Charlie down here, and did so this January. He was on clomipramine for about a week after the move, and there hasn't been a single instance of him urinating outside of the box, and I haven't given him the medication since.
best of luck, and hang in there!