This church-based b&b is a good option if you are on tight budget (I think I paid Tshs 30000/night in June 2012), want a quiet place, and are prepared to go a bit off the city centre. It is quiet, green and friendly, and has secure parking. The location is nice next to the lagoon, although there is no beach. My room was fairly big with own bathroom. If you need to leave your car here for an extended period of time, they will let you do so for a small fee. Taxi from city centre was 10-15000 Tshs.
The place is difficult to find - it is off Nelson Mandela Rd, close to fuel depots. Turn off Nelson Mandela, drive past TEC Kurasini Catholic Conference Centre, cross the railway line, and follow the road to your right until you get there.
Phone them if you don't find the place +255 22 2851140 / 2851362, email email@example.com