Modern Design Rings  

What kind?
Metal? Stone? I see lots of neat vintage jewelry on eBay, particularly silver.
Check out popgloss for contemporary offerings.

Try Pugster.com
I'm loving their Stainless Steel stuff. Amazon has a good line of SS pieces too.

Glassartist
Please post a picutre of your ring that you made for your fiancee. That was classic.

Teno
I think Teno has some lovely simple pieces in titanium and gold. Not terribly expensive either.
http://teno.com/rings.html

Its a huge area with a lot of...
Its a huge area with a lot of choice but Georg Jensen produced a lot of nice modernist silver jewelry thats still available today as are some of Bjorn Weckstrom's designs for Lapponia.

Belgian options..
In Belgium, Both Christa Reniers and Wouters & Hendrix are popular for quality modern pieces. But I am not sure which price range you are looking at ?
1 Christa Reniers (CR)
Some of the less expensieve Christa Reniers rings are in sterling silver (without any stones), priced around 250 EURO. She 's got a webshop. I got a silver CR ring from my folks when I graduated from uni. I Still love & wear it.
http://www.christareniers.com/

2 Wouters & Hendrix
More...
2 Wouters & Hendrix
More flowery designs, often asymetrical, often with stones.
(PS Hate their web design)
http://www.wouters-hendrix.com/

Thanks LS
Teno is very interesting.

3 cheaper: buy directly from...
3 cheaper: buy directly from artists (via Etsy)
Etsy is the Ebay for handmade goods & artisans. A whole lot of junk, but some great finds at reasonable prices as well.
Both housewives and graduated jewellers (and everything in between) sell there. The problem is locating something you like...
http://www.etsy.com/shop.php?user_id=79037

alternatively:
go to notcot/ notcouture and perform a search. should give you an impressive overview of modern jewellery available.
http://www.notcouture.com/

Maybe Sundance?
You might try sundancecatalog.com. They have many interesting things at different price points. I'm unsure if you are looking for something fancy like an engagement ring, or a more casual fashion piece. Sundance could work for the latter.
I also second Etsy. But it can be very difficult to search.

Modern silver style
I sell silver from India on eBay and I try to push for styles that are modern as well as more traditional or ethnic.
I don't suppose I can link here but if you want to PM me then I can let you know my ID !

Betty Jäger
A jewelry designer from Minneapolis - http://www.3jagdesign.com/
she has some beautiful modern designs.
http://www.3jagdesign.com

Riki
I restored the image to my modern love thread, but the ring makes me cringe a little (it was partly a joke after all) so I don't want to drag it along further.