Bees are some of our best pollinators, and 30% of our food supply is dependent on bees for pollination, as well as the fruits and flowers in our home gardens. Right now the Honey bee is in decline due to mite infestation, however, this bee is not our only pollinator - other bees, known as "Solitary" bees are just as hard working but because of their small size they go unnoticed and due to lack of knowledge about these fascinating insects, are greatly under appreciated. In most cases, mention the word "Bee" and most people will be squirming, recalling tales of bee stings and thus labeling all bees as bad. Unfortunately the bees that do sting, such as the wasps, hornets, yellow jackets and occasionally the honeybees, give these other Solitary bees a bad name and instantly cause fear. In most cases bees will ONLY sting if roughly handled.
The Solitary bee, depending on the species can nest either in the ground or in holes in wood, such as those made by wood borers, or in handmade nest boxes. 85% of all bees are "Solitary" and they are called this because they do not live in hives and do not have a Queen. Instead they will nest in their holes, one female per hole, but will live alongside other bees.
Some of the different species include - Orchard Mason bees (known as Mason bees), Leafcutter bees, Aphid bees and Carpenter bees.

Boxes can be homemade with 6" deep blocks of wood but to help promote healthy populations of Mason bees the use of routered trays is definately the best type of nest box to use. These come apart in layers (photos above) so the cells can be checked for parasites and cleaned out at the end of the season.
Short of this, a basic way to make a box is to take a 4x4x6 piece of wood, (remembering that a 4x4 really isn't 4x4, but more like 33/4 x 33/4) such as pine, fir or cedar, (nothing treated or waterproofed). Along one side of this block and with a 5/16" drill bit (the preferred size for Mason bees), you will drill rows of holes about 31/2" deep but not going all the way through. Holes should be drilled as smooth as possible for the bees so take your time in making the holes. They should also be spaced 3/4" apart from the center of the holes. Thus, a 4x6" would give you about 12 rows across and 4 rows down, giving a total of 48 holes. However, with holes only at a depth of 31/2" this would produce mostly male bees as the shorter the hole, the more males. More females are preferred so another way to do this would be to take the 4x4x6 and drill the holes into the end of the wood block, going in about 51/2" deep. You would fit 4 rows across and 4 rows down, giving you 16 holes. This would produce more female bees which is encouraged, so holes between 4-6" deep is recommended, but either way would still attract them.
There is one other thing that some people do and that is to use paper straws inserted in the holes. This way the holes can be cleaned out before new eggs are laid and cocoons can be cleaned to prevent mite build up. More information can be found regarding this method in some of the links below.
Once finished, place the box in an East or preferably Southeast facing position. They like the morning sun which enables them to fly early and collect pollen. You can attach your box under the eves of your house where it is sheltered, or attach it to a stake and place in the ground among preferably native plants. Plants that have been hybridized to not produce much in the way of pollen and nectar, having been bred for showy blooms instead, so try and stick to the native plants or old varieties. The height would be approximately 3 feet off the ground to avoid water from splashing up. Also tilting the box downwards slightly will help to protect from rain. You can also place a roof on the top of the box to shelter it. Make sure you place your box in a spot where you can observe the bees at work but in a spot where it will not be disturbed or knocked. Do not worry about standing near the box while they are nesting - they will fly around you and pay you no attention!
Finally - DO not place near plants that have had pesticides sprayed on them and do not use any pesticides or herbicides until at least after the bees have finished nesting.

Unlike Mason Bees which are blue-black in color, Leafcutters are similar in size & color to honeybees, with stripes going across their abdomen in shades of black, grey and tan. They do not collect pollen on their legs like honeybees but instead the females have stiff bristle type hairs on their abdomens designed to trap the pollen from the flowers they visit, coloring them yellow. The males do not have these hairs and his font feet are white.
Soon after mating the male bee dies, the female living for approximately 2 months to lay eggs. Like the Mason bee she will nest in pre-made bee houses with holes from 4"-8" deep and will make 12-15 cells, each cell lined with approximately 15 pieces of leaf with a loaf of "bee bread," a mixture of pollen and nectar placed in each cell, on which an egg is laid. They will make one cell per day on average. The larva will hatch after a few weeks and feed on the bee bread left for them. They will then turn into a pupa and depending on the species, & if eggs were laid early enough in Spring they will hatch a few weeks later. If the eggs are laid in late spring/early summer they will remain as a pupae through the remaining summer and winter until the following spring. They will then pupate, chewing their way through the leaves and emerge as adults about 3 weeks later, the males which are in the front cells emerging first, followed by the females in the deeper parts of the nest. The cycle will then start again.

The Native Leafcutter Bees are excellent pollinators of Alfalfa and one particular small species of Leafcutter bee was introduced accidentally into North America from Europe which does this job very well. These bees are used to pollinate crops of alfalfa, each single bee doing the job that would take many honeybees to do. They also have the advantage that they do not seem to get the mite problems that the honeybee does and they do not have the painful sting like that of a wasp, hornet or honeybee, which is good when humans are working in confined areas such as greenhouses alongside pollinating bees.