The 8 pin ATTiny45 is a very small but powerful controller. However, if you need an external crystal, there are only very little I/O pins left. The current version of Objective Development’s Firmware-Only USB Driver offers a new option: It can be compiled to run with 16.5 MHz +/- 1% clock. This clock rate and precision can be met with the ATTin45’s internal RC oscillator.
EasyLogger is a simple example which does not require any host software. It presents itself as a USB keyboard to the host and automatically enters the values it measures. To get a series of measurement values, just open a text editor or even a spreadsheet like Excel and press the start/stop button on the logger. EasyLogger will start to type values directly into your host application.
A single sided EAGLE PCB layout can be found at http://olgatronic.free.fr/?p=3. Click on "Fichiers Eagle" below the image.