Willows (salix) is a large family of 400+ species, many of them native to Europe. I think these are worthy of special consideration as 'plants for bees' because of the mass of pollen and nectar they produce in spring when they are covered in catkins. You may think that willows are too big for most gardens and certainly many of them are large trees but there are plenty of smaller ones too or they can be managed to keep them short by pollarding.
The common UK goat willow (or pussy willow), Salix caprea, is officially ranked as a shrub and will grow to a maximum of 10m (30 ft) but if you chop it back to a stub of trunk, c. 1m high, every other year then it produces lots of fresh new growth which has attractive bark, and will not get above 3m. Similarly with the violet willow, salix daphanoides, which as the name suggests also has a purple bark which is nice in winter.
The catkins are actually the flowers and trees will either produce all male or female catkins. The male catkins produce the pollen and you will see the catkin turn from silky grey to a yellow fuzz like the picture above.
We have planted a variety of willows in our site at rosybee both to provide spring food for the bees but we also pollard them hard to provide winter colour from thier new stems.