As part of my clock project (yet to be documented) I needed an accurate real time clock (RTC). I selected the Maxim(Dallas) DS1307. I based the decision mostly on it's popularity and because of the availability of C libraries for the AVR. Sparkfun has breakout board that's similar to this but i didn't feel like paying $20 plus shipping for such a simple board (plus I think that Don's parsimony is rubbing off on me a little). In addition I wanted integrated pull-up resistors in the i2c lines.
Bill of materials:
Digikey part#, item, price, quantity
- 300-8735-1-ND, 32.768 kHz crystal, $0.84, 1
- DS1307Z+-ND, DS1307 RTC, $3.29, 1
- BH32T-C-ND, CR2032 battery holder, $1.04, 1
- P189-ND, CR2032 battery, $0.28, 1
- 478-1831-ND, 0.1 uF tantalum cap, $0.46, 1
- 2.0QBK-ND, 2k ohm carbon film resistor, $0.27, 3 (these are what I used, you can substitute whatever 1.8k-20k axial resistors you have laying around)
- A36390-ND, 5 pin male header, $0.91, 1 (I just used some make headers I had laying around,
I just found the equivalent on digikey for completeness sake. digikey/mouser/newark all have terrible prices on pin headers)
total= $7.57
board supplies:
- single/dual sided copper PCB
- Toner transfer paper (digikey# 182-1003-ND)
- green toner reactive foil (TRF)(digikey# 182-1021-ND)
- Ferric Chloride etchant
parts placement: 
top side after assembly: 
bottom side after assembly: 
PDF of etching pattern for the board is attached to this post. the libraries for this part for C and Arduino are all over. just google.
If I feel like it, I'll make another one that has jumpers on the battery and pull up lines, so you can clear the memory or turn off the pull ups if you want to. I'll probably rotate the battery to make the board smaller too. right angled header pins are another feature to incorporate, but i didn't have any on Sunday when i built it
for prototype purposes this will work just fine for now.