OP is not talking about a friend with cats. S/he is talking about people who have cats and are "cat lovers." Those of you who simply have cats and would find a cat-themed gift weird are probably not the target audience for this question.

For example, at my cat's birthday party, my friend brought us a cat-themed welcome mat, and we liked it very much.

I have cats and they are very important to me. I would have thought something for the cats or with a cat theme would be sweet. If you want to stick to wine but also want to have something related to cats try cat's pee on a gooseberry bush Sauvignon blanc wine. I thought it was good wine (although I know very little about wine)-it at least tasted good and I got a good laugh out of the name.

i have 2 cats. never gotten a gift. but a small toy is cute.

Op, how about a modern/artsy looking serving dish, salt and pepper shakers, vase, statue, check pier one, world market, etc. or for something inexpensive you could do a cute cat coffee mug with some good tea or coffee. If you are getting a gift for the actual cat, how bought a nice cat bed or scratching pad.

I have a cat, and would find a cat themed house warming gift very odd.

I would vote for a cat-themed picture frame, if they are truly "cat lovers." Not sure about toys, since cats can be picky.

I had two cats after college and when we moved a friend (also with two cats) gave me a cat gift basket.
She put fun treats, some toys, a cute collar etc- all for my cats only (so no cat picture frames or anything). I imagine she got everything from target (so nothing crazy, I think it was like 3 different flavors of temptations for example) but it was just a fun gift and I loved that it was just for my cats. I still remember it. It was thoughtful.

Cutesy stuff with cat photos/images is weird. Toys, scratching posts and other useful stuff is a great idea. Modern Cat is a really good website but pricey. I also really like the humor behind products from Fat Cat Inc.-they have a lot of toys that are designed to make humans laugh and the products are widely available. You didn't give a price range but I would like it if somebody gave me this:

http://www.wag.com/cat/p/fat-cat-big-mamas-scratch-n-play-ramp-for-cats-with-catnip-434309?site=CA&utm_source=Google&utm_medium=cpc_W&utm_term=FPS-19808&utm_campaign=GooglePLA&CAWELAID=1418947985&utm_content=pla&ca_sku=FPS-19808&ca_gpa=pla&ca_kw=%7Bkeyword%7D

wag.com is another good source for pet toys.

I think it would be really sweet. You know your friends best, OP.

I've had cats before, and I think it's weird to get cat themed stuff.

If I were on the receiving end of a cat themed house warming gift, I wouldn't think it was odd at all. Better than another bottle of wine.

My MIL gave us these and our cat loves them.

http://www.amazon.com/Yeowww-Catnip-Toy-Yellow-Banana/dp/B000AUJFHE

Also, and this is nicer because at least in our case, it came with a bow around it, this cat mat:

http://feralcare.org/catnip-mats.html

Catnip plant?

Do you know these folks want a cat-themed housewarming gift? We have cats but a cat toy as a housewarming gift would be.... odd.

I would encourage you to bring a nice bottle of wine instead. Definitely steer away from plants (another traditional housewarming gift) as many are toxic to cats.

What would be a good housewarming gift for cat lovers without kids but have 3 cats? Cat toys? If so, any particular type? I have zero knowledge on cats so any suggestion would be helpful. Thanks!