According to their website, Seventh Generation powders are not recommended for use in HE/front-load machines. Tell me not to do something and I will. And I did. I used the Free and Clear powder in my front loader and it worked just fine. On regular and large loads I used the quantities recommended on the box and didn't have any issues with excess suds. On smaller loads, though, I had to cut back the dosage to keep sudsing in check.
Right now I'm working down a box of Whole Foods 365 Laundry Powder and it appears to be very similar to Seventh Generation powder. Same manufacturer? At any rate, it works just as well as the S.G., and only costs about $6.99 for a 60-load box. So far I haven't had any suds issues with it either.
As for other brands, Ecover, Bi-O-Kleen, and Country Save all make powders suitable for HE machines. I have a box of Ecover, and it's so-so. Fine for everyday lightly soiled things, but lacks the muscle to attack stains. I haven't tried Bi-O-Kleen powder, but did use Country Save years ago. IIRC, it worked fine. Country Save makes an ecological powdered oxygen bleach that I really like, and used in conjuction with ecological detergents it boosts the cleaning power considerably.