Below are some journals (just one publisher so far) that (very) frequently solicit my expert opinions, (some) without really checking whether I have the expertise or not, and without any impact factor.

Resources I use to check the credibility of unknown journals:

"Proper" invitations

Journal of Artificial Intelligence Research, 31May 2012, ISI Impact (2011) 1.143, ISSN: 1076-9757. Editor: Alice Lopez

Hindawi Publishing Corporation

Founded in 1997 in Cairo
. currently employs more than 300 employees
. Publishes 200+ open access, peer-reviewed journals.
. Open-access is financed by author fees i.e. non-subscription based.. The author fee currently "tops out at $1,500"
. recently partnered with Sage, a well-established e-journal publisher

22 of the 300 journals have an impact factor higher than 0 for the printed (not online) versions, but below

Some other researchers impressions: http://list.uvm.edu/cgi-bin/wa?A2=MEDLIB-L;wGArog;20110726180255-0700d

I have been solicited to write an expert article for the following journals the past year. Even though I feel flattered, I would not think too much of researchers that publish in these journals with zero impact factor (and not just because they are too new to calculate the factors!)