I own the houses in whiterun,riften and solitude.

Is there any other cities that have houses for sale?

Windhelm, but you must complete Blood on the Ice and progress through the Civil war far enough to be offered it.

Markath,do the task for the Jarl (retrieve a shield I think)

With Hearthfire, Falkreath, Dawnstar and Morthal also have houses you can build, though they are outside the towns themselves.

There's a house in Raven Rock too, if you have the Dragonborn dlc.

I went out and grabbed all the houses with my first character, just because. Then I spent the rest of my game trying to remember where I put stuff. And this was before Hearthfire. Now, I get one or two houses per character (at least to use, I may acquire other houses but I never use them) and now I can find my stuff.

lol I did the same thing. Now the only problem I have is remembering which characters uses which houses out of the gazillion home mods I have installed.

Yes, I'll often take time to "consolidate," by taking all of the stored items from all the other houses and putting them all in one location. Otherwise, yeah, you end up wander everywhere trying to remember where on earth you left that one amulet.

It's stuff like that that can really test the limits of a person's "No fast travel" rule, lol, cause it can sure feel depressing and irritating when you go trekking across all of Skyrim just find out that the home you thought you left the amulet in isn't the right one after all.