For cleaning windows and mirrors around the house, I usually just use a (reasonably) clean rag and give the glass a good dry scrubbing. If that doesn't work, or if the surface is unusually dirty, I'll just do the same with a damp rag, and again with a dry rag. This works pretty well, and doesn't leave any noticeable streaks.
For the tank, a lot of times I'm dealing with hard water spots/scale. If the above doesn't work, I'll just use a spray bottle with some vinegar.
Vinegar seems to work pretty well for me, it's cheap, and generally useful.