I was usually working with Canon lenses (the 85mm f/1.2, 8-15mm, 16-35mm), but recently, I've got myself the Zeiss 100mm f/2, and it blew me away. I never thought it possible, but it, while making pictures look expensive, gives them a completely different feel compared to Canon. I want to find more lenses which would provide me a look different to Canon or Zeiss.
I prefer primes, auto focus is not important. I'm a Canon user (5d3), and the lenses are used mostly to take photos of people. What matters to me is that the picture quality is really good, and that it should look distinctive from what I've already got.
Which other lenses should I have a look at?
Have a look at the Lensbaby Composer (update: discontinued): It has an unique feel to it, has been used very effectively by wedding and portrait professionals for a long time. Now replaced by the Lensbaby Composer 2, quite a bit more expensive.
At ~$300 for the Canon EOS mount version, it isn't cheap for a lens that isn't really general purpose at all, but for sheer creative possibilities, it is unmatched.
At a much lower price point, some very striking portraiture has been done by simply stretching a single layer of the finest available black sheer nylon stockings, with or without a strategically located hole in the material, over any lens you have, and using the resultant partial soft focus to powerful effect. This is especially useful to tone down beard stubble, harsh wrinkle lines or double chins.
Some studio photographers use very long telephoto lenses (300mm, 500mm) for portraiture: The angles and structural curves of faces appear stronger and better defined, and depth of focus can be finely tuned for interesting effect. Using a tele with a very deep DoF gives an almost 3-dimensional feel to very tightly framed portraits - You would need lots of light, though, to cope with the really narrow apertures required.
The Leica R series SLR lenses are very highly regarded and can be mounted relatively easily on a Canon DSLR. If you already have a 100mm lens I'd suggest the Elmarit APO 180mm f/2.8, these don't come cheap, however.
There are a lot of cheap fast 55mm and 58mm lenses that give a very unique rendering due to simple designs and old fashioned coatings. Have a look at the Russian Hilios 58 f/2.0
There's also the manual Canon FD telephotos that can now be effectively mounted on an EOS thanks to a guy callyed Ed Mika. The 300 f/2.8L is relatively cheap (much less than a Leica) and great for long distance portraits.
Well, if you want to go all-in, you could always get a bellows or a 35mm-dedicated view camera body and avail yourself of the Sinar, Rodenstock and Schneider offerings. Most of them are f/5.6 wide-open, so you're not going to get razor-thin DoF, but they're sharp as all-get-out, contrasty in a way that's closer to the look of the Zeiss and Leitz lenses, and have a large enough image circle to give you significant movements (so you can do the Brenizer thing without having to worry about straightening distortions before stitching).
You can also mount some of the great old enlarger lenses, like the Fujinons, in a Sinar or Copal shutter—they're great at anything significantly closer than infinity. Again, "people length" lenses cover considerably more than a 35mm frame, since they were designed to be non-vignetting on 6x6 and larger negs.
You do sort of have to develop a view-camera state of mind—frame, focus, cock the shutter and then fire—if you want to use the in-lens shutter (which is triggered by the flash sync—you'd actually fire flashes from the lens's PC terminal). You can also work in live view if you prefer to work "live" and use the focal plane shutter.