It kind of depends on what sort of price range and purpose you're buying for, IYSWIM.
I'm pregnant with my third at the moment and there is a 10 year age gap between this one and my last one - I have to say that, in those ten years, maternity wear from the high street has definitely improved beyond measure! When I was expecting my DS (now 10), the only stores I could buy maternity wear in were Dorothy Perkins and C&A! Blooming Marvellous was around then too but only via mail order. It used to be so frustrating, so I'm quite pleased to see how things have changed now.
For me, I've really liked the following;
For work wear (black trousers, skirts, shirts tc), Next has been good.
For dresses or clothes for a slightly more dressy look, Blooming Marvellous and Picchu.com have been good.
For every day wear, Dorothy Perkins is great.
For items that are a bit more 'on trend', TopShop is excellent. For similar stuff that's more reasonably priced, you can't beat H&M and New Look.
If you can afford a bit extra, Isabella Oliver is good as are Formes and Seraphine.
For maternirt jeans, I would whole-heartedly recommend bootcut 'on the bump' jeans from mamas and pappas - the best ones I've ever had.
As has been said already, there are lots of swing/trapeze/empire line shape clothing in the shops at the moment - you could easily buy tops in these shapes that aren't maternity wear (perhaps in a bigger size if needed). I know I've already spotted quite a few fab tops in Oasis that I'm hankering after now!
A couple of health warnings on top of all of these recommendations!
Next - not all of their stores stock the maternity range so do check before you turn up at your local branch. Alternatively, you can buy direct from their website instead.
Vertbaudet - trousers aren't so good if you're not particularly tall nor long of limb. I'm 5'5" and had to send regular length trousers back when they trailing several feet of material over my feet and onto the floor!
ebay - do be careful when you buy stuff from here. Always check that the photos featured on the site are of the actual garment that you'll be buying and not taken from the site from where they were originally bought - you might end up disappointed when something a bit grubbier than you were expecting arrives in the post!
Maternity trousers generally - try to buy 'on the bump' styles if you can, (especially if you're not particularly tall), rather rhan over the bump or under the bump styles. In my experience, on the bumo trousers are more likely stay up and in place when worn!