Many types of plants can be grown indoors on windowsills, from flowering plants like African violets to herbs like basil and thyme. All you need is a windowsill with suitable light; some plants like direct sunlight while others prefer indirect light, and some like shade. Indoor windowsill plants need a little extra care, too, including regular watering and protection from drafts and the colder temperatures that sometimes occur in winter next to a window, especially if the window is not well insulated.
Most windowsill plants are grown for their visual appeal--flowers or foliage, or both. Some African violets, for example, have variegated leaves that add as much interest as the blooms do, while ferns are grown for their attractive fronds. Other plants have practical uses. Herbs like chives and oregano can be used in cooking, and juice from the aloe plant may help heal damaged skin. If you can't wait for spring to come, you can give it a jump start by planting bulbs like tulips and daffodils in pots and putting them on a windowsill; the warmer air inside will force the bulbs to grow and flower well before those outside do.
Windows that face south or southeast generally provide the most sunlight. Plants that like at least four or five hours a day of sunlight include herbs, 'calla lily' begonias, and dwarf varieties of gardenias, which add both beauty and fragrance to a room. The Christmas cactus and ferns like the maidenhair fern, on the other hand, need light but not direct sun; they could be placed in a north-facing window in the summer and an east-facing one in the winter. A shade or partially closed blind can also be used to provide indirect light in front of a sunny window. Some kinds of orchids can even be grown on a windowsill, including the phalaenopsis or moth orchid. It doesn't need much light at all, so a north-facing window or one that's shaded by a tree or an awning would be perfect.
Potted plants depend on you for regular watering; unlike plants outdoors, they can't extend their root systems in search of water. Most don't like to be kept wet; let them dry out slightly between watering. When the top two inches or so of soil in the pot feels dry, water the plant until water comes out the bottom of the pot. Pour the water out of the saucer underneath the pot so that it doesn't stand in water. Orchids like high humidity but little watering; water them only every two or three weeks. In the meantime, you can mist them a few times a day with a spray bottle, or put them on a tray with pebbles in the bottom and keep water in the tray up to just below the top of the pebbles. The water will evaporate and provide humidity, but the pots and growing medium will stay dry.
Since indoor windowsill plants are meant to provide visual interest, choose attractive pots with matching saucers. Small potted plants are usually sold in plastic pots; when re-potting, it's best to choose a pot only one size larger than the one the plant came in; that way it won't expend all its energy putting out an extensive new root system. In addition, of course, it's important to make sure the pot and saucer fit comfortably on the windowsill where you plan to put them.
Soil and Feeding
A good commercial potting soil is fine for most indoor windowsill plants. To make sure it drains well, you could add sand or vermiculite, sold in most garden stores. Although it may be tempting to use soil from your garden, you risk bringing in pests or diseases if you do so. You can buy organic potting soil if growing organically is important to you. Orchids need a special potting mixture consisting of wood chips, so you should take care to buy that mixture instead of using regular potting soil. A good all-purpose plant food is fine for most indoor plants; you can just follow the instructions on the label. Organic plant food is also available. However, African violets and orchids need specially formulated fertilizer.