Components such as crystals, capacitors, and other semiconductor devices are usually required in microcontroller based applications. Here are some links to manufacturers and distributors that may be helpful in your projects:

Raltron has any sort of crystal, resonator, or oscillator you might want. Fox is another good supplier of these parts.

Dallas Semiconductor supplies all sorts of interface circuits, reset and watchdog parts, security enhanced products, real-time clock chips and much more.

Maxim Integrated Products is famous for the MAX232 serial tranceiver chip. They offer a variety of innovative interface and wireless communication parts.

DigiKey is a leading mail-order distributor that sells in almost any quantity. An ideal source for the hobbyist for Microchip parts as well as almost any other component.

Murata is a leading manufacturer of capacitors and other passive components and piezoelectric devices including buzzers and resonators.