Adafruit 12 MHz Ceramic Resonator / Oscillator [ADA1874]
Keep your microcontroller tickin' with a ceramic oscillator. These little devices are bread-board friendly (0.1" pin spacing) and easy to use. They act much like crystals with the 'stability' capacitors inside. They have 0.5% accuracy, great for every day electronics and microcontroller work and certainly precise enough for UART or other baud-rate generation. For ultra-high precision needs (Certain USB timing, long-term time-keeping, RF/PLL/VCO driving) you may need a crystal or temperate-compensated oscillator.
To use, connect the two outer pins to XTAL-in and XTAL-out on your circuit (they package is symmetric so use whichever outer pin) and the inner pin to ground. Check the datasheet for more specs