Yellow Line which travels through southern & western Stockholm and the Blue Line which takes you through the northern & eastern parts of the city. You can also purchase a combination ticket for both lines. This is a perfect way to combine a guided tour of the city with visits to most of Stockholm's most interesting sites and venues. If you prefer your bus tours to be stop-free, then there is the Panorama Bus Tour which runs year round.
There is still too much ice in the harbor for the city sightseeing boats... they will be starting up in the first weeks of April. In the meantime you have the Stockholm Winter Tour (daily) and Little Archipelago Tour (weekends), two boat tours that take you out into the inner Stockholm archipelago.