Might be able to help out here.
I've got a BifferBoard which I use as a NAS along with loads of other stuff running on it which, like the Rpi, has no RTC.
The way around it is to setup the NTP daemon to grab the time on boot then setup a cron job to update the time every 24h.
Looking at my NAS its uptime is 131 days (just over) and the time is bang on.
Obviously won't work with the non networked Rpi.
For those, I guess the easiest clock source would be the DS1307 as it's available in a DIP package. However, it requires an external crystal and isn't temperature compensated which, from experience, leads to serious drift in the time, so bad it makes it almost useless.
So it would be better to use the DS3234 which, as pointed out above, has an integrated crystal but, more importantly, is also temperature compensated so keeps time much better. However, it's only available in an SMD package.