Whilst updating myself with all the various projects, I came across this one which seems like a nice little "earner".
The project website is here:
http://wuprop.boinc-af.org/
The WU's are approx 6 hours long, but on the upside is that it is non-CPU intensive which means it can run as an extra task along with whatever other tasks you are running.
So, on my dual-core I can run 2 other tasks PLUS WUProp (making 3 in total) and on my quad-core I can run 4 other tasks PLUS WUProp (making 5 in total), and ALL running at the same time.

Credits are a bit low, but basically the effort required is zero, so any extra credits you get are basically for free.
Give it a go - you've got nothing to lose !!
regards
Tim