I have tried Monster and Career Builder with little success. I had a interview with a very large organization last Monday in Des Moines but it was a 60/40 position with the 60 being grant writing and the 40 being something that I wasn't qualified adequately. I am sure there must be a nice site somewhere that has that magic position for me that pays $100,000.00+ with a four day work week?
You can find some grant writer positions on Hot Jobs, though you have to wade through a lot of spam postings. I would recommend Idealist.org.
If you are not already a member of the For GrantWriters Only LinkedIn group, you may want to join. I list the reputable looking positions I find on there. There are two other LinkedIn groups (Non Profit & Philanthropic Job Board, Nonprofit Job Search Network) that may be helpful.
I would be very careful about the 'wanted grant writer' requests on Craigs List - many seem desperate and others are questionable. There a few that seem very legitimate.
There are freelance sites that you may consult such as Guru.com, elance.com, and sologig.com
I know Melanie already mentioned indeed.com but I thought I would mention something else about this same site. I have a search saved at this site and it emails me the results weekly (I think you can change the frequency). The cool thing about indeed.com is that it is a meta-search of all the major job sites (i.e., Monster, etc), so you get listings from many different sites by just registering with this one. I've also found salary.com, ifreelance.com and particularly elance.com helpful. The latter two are most helpful if you - like me - are a consultant. Hope these help you out.
I agree with looking on Idealist for non-profit job openings. Also try simplyhired.com, an aggregator site. The Ladders specializes in 100k+ jobs, but it costs money.
I also find idealist.org to be the best. Sometimes AFP has listings as well. You can set up RSS feeds for both so they feed right into Google Reader or whatever reader you use.
Have you tried Indeed.com? It posts jobs from a variety of sources and I've had pretty good luck there although I haven't found that elusive 4 day a week $100K job :). It will even send you updates when new grant writer jobs are posted.
In addition to those mentioned, I would also look at the Chronicle of Philanthropy - there are sometimes grant specific positions listed. American Association of Grant Professionals also posts job listings at http://careers.grantprofessionals.org/search.cfm
I've utilized many of the sites that other posters have mentioned. I guess my biggest question is what kind of 'luck' has everyone had? I mean, I even had the opportunity to discuss a grant writing opportunity from Linked In, but it never panned out due to budget issues on their end.