I trekked twice, once just myself and guide/porter, second small group of friends and same guide w/additional porters.
We used World Peace Trekking, Kathmandu, Lal and Sinjan Lama. Not sure if they are still in business (as I haven't received letters requesting US sponsorship in some time
).
I trekked twice in the Annapurna Region, to Annapurna Sanctuary (awesome) and the Full Round Annapurna Circuit (pretty great). From what I've read, there are other ecotreks that have cropped up around the ACAP (Annapurna Conservation Area Project region) since 2000 and 2001, when I visited Nepal.
I visited the Sanctuary in March and there was still a great deal of snow to deal with an avalanche risk beyond Chomrong. I did the circuit counterclockwise in November, and saw NO snow.
On my first visit, my "guide" kept asking to read my guidebook each night, and I soon realized that I was funding the vacation of two brothers, "internal tourists". All was well, we had alot of fun, they interpreted for me and bargained rates for me -- and I gave them a job for ten days. On my second visit, they arranged a certified climbing sherpa guide to take us all round Annapurna and the brothers served as porters. This also worked quite well. There are hundreds of young men in Kathmandu that are looking for guide work. If you enlist one, just be kind enough to spend a few of your own bucks before leaving Kathmandu to ensure they have the essentials. And its also a good idea to carry your own money; and keep some small rupee denominations in easy to reach areas for each day on the trail you will inevitably be politely shaken down for a school building project. Consider it part of the experience.
Its actually quite fun.
We always stayed in tea houses, sometimes in just a villager's home or outbuilding. Waterless hand cleaner will become your friend.
If you are confident, and enjoy cultural immersion, its pretty safe to go the scaled back Kathmandu Nepali based guide route. If you feel you need the security of a full service trek, you will find them too.
Whatever you decide, have a great time and please post pictures. (I've been planning to digitize my slides for years).
www.Lonelyplanet.com is a good source of info.