medium size tree for my front yard to provide good shade?

A Japanese "weeping" Cherry such as Kiku-Shidare Sakura or Snow Fountain would be a good reliable choice with the added bonus of flowering blossom in spring.

http://landscaping.about.com/cs/treesshrubs/a/weep...

Where do you live? A tree that grows well in the Pacific Northwest will not do well in the Southwest for example.

Your best bet is to contact your county extension agent (you can find them on line by googling your county's name), research what they suggest for medium sized shade trees, talk to employees at local nurseries, take a walk around your neighborhood and see what other people have planted (gardeners love to talk about their gardens).

The deciduous crape myrtle is among the longest blooming trees in existence with flowering periods lasting from 60-120 days. Crapes come in heights as short as 18 in (46 cm) and as tall as 40 ft (12 m). Leaves are alternate and smooth, but leaf size depends on variety. Flowers are borne in summer in big showy clusters and come in white and many shades of pink, purple, lavender and red. The fruits that follow are brown or black. When mature they dry and split releasing disk shaped seeds. Depending on variety, crapes grow as large shrubs or as trees that may be either upright or spreading. Large varieties are very fast growing and can put on several feet in a single growing season. Many types have interesting bark that exfoliates in thin flakes exposing lovely cinnamon or gray inner bark. Crapes tend to produce many suckers that should be removed as they appear if you want to maintain them as trees with distinct trunks. They are enthusiastic reseeders so you may find yourself pulling up baby crapes throughout the summer..

Lindens are good... I have a Littleleaf Greenspire Linden :) They usually grow to about 35 feet, with a 20-30 foot spread. They have a nice pyramid shape, too. They turn yellow in the fall, and have spring flowers that smell good.

Bradford Pear. Grows fast, nice shape, and flowers in the spring.